diff --git a/core/android/account/src/main/kotlin/net/thunderbird/core/android/account/Identity.kt b/core/android/account/src/main/kotlin/net/thunderbird/core/android/account/Identity.kt
index b35fe50da6b..6d16862d414 100644
--- a/core/android/account/src/main/kotlin/net/thunderbird/core/android/account/Identity.kt
+++ b/core/android/account/src/main/kotlin/net/thunderbird/core/android/account/Identity.kt
@@ -10,11 +10,13 @@ data class Identity(
     val email: String? = null,
     val signature: String? = null,
     val signatureUse: Boolean = false,
+    val signatureIsHtml: Boolean = false,
     val replyTo: String? = null,
 ) : Parcelable {
     // TODO remove when callers are converted to Kotlin
     fun withName(name: String?) = copy(name = name)
     fun withSignature(signature: String?) = copy(signature = signature)
     fun withSignatureUse(signatureUse: Boolean) = copy(signatureUse = signatureUse)
+    fun withSignatureIsHtml(signatureIsHtml: Boolean) = copy(signatureIsHtml = signatureIsHtml)
     fun withEmail(email: String?) = copy(email = email)
 }
diff --git a/core/android/account/src/main/kotlin/net/thunderbird/core/android/account/LegacyAccountDto.kt b/core/android/account/src/main/kotlin/net/thunderbird/core/android/account/LegacyAccountDto.kt
index 3018387a95d..40111a68997 100644
--- a/core/android/account/src/main/kotlin/net/thunderbird/core/android/account/LegacyAccountDto.kt
+++ b/core/android/account/src/main/kotlin/net/thunderbird/core/android/account/LegacyAccountDto.kt
@@ -433,6 +433,15 @@ open class LegacyAccountDto(
             identities[0] = newIdentity
         }
 
+    @get:Synchronized
+    @set:Synchronized
+    var signatureIsHtml: Boolean
+        get() = identities[0].signatureIsHtml
+        set(signatureIsHtml) {
+            val newIdentity = identities[0].withSignatureIsHtml(signatureIsHtml)
+            identities[0] = newIdentity
+        }
+
     @get:JvmName("shouldMigrateToOAuth")
     @get:Synchronized
     @set:Synchronized
diff --git a/feature/account/storage/legacy/src/main/kotlin/net/thunderbird/feature/account/storage/legacy/LegacyAccountStorageHandler.kt b/feature/account/storage/legacy/src/main/kotlin/net/thunderbird/feature/account/storage/legacy/LegacyAccountStorageHandler.kt
index 8c8d0488a16..0d994597cd4 100644
--- a/feature/account/storage/legacy/src/main/kotlin/net/thunderbird/feature/account/storage/legacy/LegacyAccountStorageHandler.kt
+++ b/feature/account/storage/legacy/src/main/kotlin/net/thunderbird/feature/account/storage/legacy/LegacyAccountStorageHandler.kt
@@ -271,6 +271,7 @@ class LegacyAccountStorageHandler(
             val email = storage.getStringOrNull(keyGen.create("$IDENTITY_EMAIL_KEY.$ident"))
             val signatureUse = storage.getBoolean(keyGen.create("signatureUse.$ident"), false)
             val signature = storage.getStringOrNull(keyGen.create("signature.$ident"))
+            val signatureIsHtml = storage.getBoolean(keyGen.create("signatureIsHtml.$ident"), false)
             val description = storage.getStringOrNull(keyGen.create("$IDENTITY_DESCRIPTION_KEY.$ident"))
             val replyTo = storage.getStringOrNull(keyGen.create("replyTo.$ident"))
             if (email != null) {
@@ -279,6 +280,7 @@ class LegacyAccountStorageHandler(
                     email = email,
                     signatureUse = signatureUse,
                     signature = signature,
+                    signatureIsHtml = signatureIsHtml,
                     description = description,
                     replyTo = replyTo,
                 )
@@ -293,11 +295,13 @@ class LegacyAccountStorageHandler(
             val email = storage.getStringOrNull(keyGen.create("email"))
             val signatureUse = storage.getBoolean(keyGen.create("signatureUse"), false)
             val signature = storage.getStringOrNull(keyGen.create("signature"))
+            val signatureIsHtml = storage.getBoolean(keyGen.create("signatureIsHtml"), false)
             val identity = Identity(
                 name = name,
                 email = email,
                 signatureUse = signatureUse,
                 signature = signature,
+                signatureIsHtml = signatureIsHtml,
                 description = email,
             )
             newIdentities.add(identity)
@@ -556,6 +560,7 @@ class LegacyAccountStorageHandler(
                 editor.putString(keyGen.create("$IDENTITY_EMAIL_KEY.$ident"), identity.email)
                 editor.putBoolean(keyGen.create("signatureUse.$ident"), identity.signatureUse)
                 editor.putString(keyGen.create("signature.$ident"), identity.signature)
+                editor.putBoolean(keyGen.create("signatureIsHtml.$ident"), identity.signatureIsHtml)
                 editor.putString(keyGen.create("$IDENTITY_DESCRIPTION_KEY.$ident"), identity.description)
                 editor.putString(keyGen.create("replyTo.$ident"), identity.replyTo)
                 ident++
@@ -577,6 +582,7 @@ class LegacyAccountStorageHandler(
                 editor.remove(keyGen.create("$IDENTITY_EMAIL_KEY.$identityIndex"))
                 editor.remove(keyGen.create("signatureUse.$identityIndex"))
                 editor.remove(keyGen.create("signature.$identityIndex"))
+                editor.remove(keyGen.create("signatureIsHtml.$identityIndex"))
                 editor.remove(keyGen.create("$IDENTITY_DESCRIPTION_KEY.$identityIndex"))
                 editor.remove(keyGen.create("replyTo.$identityIndex"))
                 gotOne = true
diff --git a/legacy/core/src/main/java/com/fsck/k9/message/MessageBuilder.java b/legacy/core/src/main/java/com/fsck/k9/message/MessageBuilder.java
index 0c077ccecdf..0b2032f0c42 100644
--- a/legacy/core/src/main/java/com/fsck/k9/message/MessageBuilder.java
+++ b/legacy/core/src/main/java/com/fsck/k9/message/MessageBuilder.java
@@ -351,6 +351,7 @@ private TextBody buildText(boolean isDraft, SimpleMessageFormat simpleMessageFor
         if (useSignature) {
             textBodyBuilder.setAppendSignature(true);
             textBodyBuilder.setSignature(signature);
+            textBodyBuilder.setSignatureIsHtml(identity.getSignatureIsHtml());
             textBodyBuilder.setSignatureBeforeQuotedText(isSignatureBeforeQuotedText);
         } else {
             textBodyBuilder.setAppendSignature(false);
diff --git a/legacy/core/src/main/java/com/fsck/k9/message/TextBodyBuilder.java b/legacy/core/src/main/java/com/fsck/k9/message/TextBodyBuilder.java
index 5e066631263..6e125f2e57a 100644
--- a/legacy/core/src/main/java/com/fsck/k9/message/TextBodyBuilder.java
+++ b/legacy/core/src/main/java/com/fsck/k9/message/TextBodyBuilder.java
@@ -5,6 +5,7 @@
 
 import com.fsck.k9.K9;
 import com.fsck.k9.message.html.HtmlConverter;
+import com.fsck.k9.message.html.HtmlSignatureSanitizer;
 import com.fsck.k9.mail.Body;
 import com.fsck.k9.mail.internet.TextBody;
 import com.fsck.k9.message.quote.InsertableHtmlContent;
@@ -20,6 +21,7 @@ class TextBodyBuilder {
     private boolean mSignatureBeforeQuotedText = false;
     private boolean mInsertSeparator = false;
     private boolean mAppendSignature = true;
+    private boolean mSignatureIsHtml = false;
 
     private String mMessageContent;
     private String mSignature;
@@ -182,7 +184,10 @@ public TextBody buildTextPlain() {
     private String getSignature() {
         String signature = "";
         if (!isEmpty(mSignature)) {
-            signature = "\r\n" + mSignature;
+            String plainSignature = mSignatureIsHtml
+                    ? HtmlConverter.htmlToText(mSignature)
+                    : mSignature;
+            signature = "\r\n" + plainSignature;
         }
 
         return signature;
@@ -191,7 +196,9 @@ private String getSignature() {
     private String getSignatureHtml() {
         String signature = "";
         if (!isEmpty(mSignature)) {
-            signature = HtmlConverter.textToHtmlFragment(mSignature);
+            signature = mSignatureIsHtml
+                    ? HtmlSignatureSanitizer.sanitize(mSignature)
+                    : HtmlConverter.textToHtmlFragment(mSignature);
         }
         return signature;
     }
@@ -244,6 +251,10 @@ public void setAppendSignature(boolean appendSignature) {
         mAppendSignature = appendSignature;
     }
 
+    public void setSignatureIsHtml(boolean signatureIsHtml) {
+        mSignatureIsHtml = signatureIsHtml;
+    }
+
     private static boolean isEmpty(String s) {
         return s == null || s.length() == 0;
     }
diff --git a/legacy/core/src/main/java/com/fsck/k9/message/html/HtmlSignatureSanitizer.kt b/legacy/core/src/main/java/com/fsck/k9/message/html/HtmlSignatureSanitizer.kt
new file mode 100644
index 00000000000..749a147ba33
--- /dev/null
+++ b/legacy/core/src/main/java/com/fsck/k9/message/html/HtmlSignatureSanitizer.kt
@@ -0,0 +1,26 @@
+package com.fsck.k9.message.html
+
+import org.jsoup.Jsoup
+import org.jsoup.safety.Safelist
+
+/**
+ * Sanitizes user-supplied HTML signatures before they are inserted into outgoing mail.
+ *
+ * Uses a Jsoup [Safelist.relaxed] baseline (common formatting tags, images, links, tables)
+ * and tightens it so scripting constructs cannot survive a round-trip through the signature
+ * field. Specifically, all `on*` event-handler attributes and `javascript:` URLs are removed,
+ * and `<script>`, `<style>`, `<iframe>`, `<object>`, and `<embed>` elements are not in the
+ * allowlist to begin with.
+ */
+object HtmlSignatureSanitizer {
+    private val safelist: Safelist = Safelist.relaxed()
+        .addAttributes(":all", "style", "class", "dir")
+        .addProtocols("a", "href", "http", "https", "mailto", "tel")
+        .addProtocols("img", "src", "http", "https", "data", "cid")
+
+    @JvmStatic
+    fun sanitize(html: String): String {
+        if (html.isEmpty()) return html
+        return Jsoup.clean(html, "", safelist)
+    }
+}
diff --git a/legacy/core/src/main/java/com/fsck/k9/preferences/IdentitySettingsDescriptions.java b/legacy/core/src/main/java/com/fsck/k9/preferences/IdentitySettingsDescriptions.java
index 1e7c9ae9939..61880cfd65a 100644
--- a/legacy/core/src/main/java/com/fsck/k9/preferences/IdentitySettingsDescriptions.java
+++ b/legacy/core/src/main/java/com/fsck/k9/preferences/IdentitySettingsDescriptions.java
@@ -36,6 +36,9 @@ class IdentitySettingsDescriptions {
                 new V(1, new BooleanSetting(true)),
                 new V(68, new BooleanSetting(false))
         ));
+        s.put("signatureIsHtml", Settings.versions(
+                new V(111, new BooleanSetting(false))
+        ));
         s.put("replyTo", Settings.versions(
                 new V(1, new OptionalEmailAddressSetting())
         ));
diff --git a/legacy/core/src/main/java/com/fsck/k9/preferences/Settings.java b/legacy/core/src/main/java/com/fsck/k9/preferences/Settings.java
index 3a740feea59..ade2857250d 100644
--- a/legacy/core/src/main/java/com/fsck/k9/preferences/Settings.java
+++ b/legacy/core/src/main/java/com/fsck/k9/preferences/Settings.java
@@ -35,7 +35,7 @@ public class Settings {
      *
      * @see SettingsExporter
      */
-    public static final int VERSION = 110;
+    public static final int VERSION = 111;
 
     static Map<String, Object> validate(int version, Map<String, TreeMap<Integer, SettingsDescription<?>>> settings,
             Map<String, String> importedSettings, boolean useDefaultValues) {
diff --git a/legacy/core/src/test/java/com/fsck/k9/message/IdentityHeaderBuilderTest.kt b/legacy/core/src/test/java/com/fsck/k9/message/IdentityHeaderBuilderTest.kt
index 59ce927a5ec..8086fc4d870 100644
--- a/legacy/core/src/test/java/com/fsck/k9/message/IdentityHeaderBuilderTest.kt
+++ b/legacy/core/src/test/java/com/fsck/k9/message/IdentityHeaderBuilderTest.kt
@@ -70,7 +70,14 @@ class IdentityHeaderBuilderTest : RobolectricTest() {
         signatureUse: Boolean = false,
         replyTo: String? = null,
     ): Identity {
-        return Identity(description, name, email, signature, signatureUse, replyTo)
+        return Identity(
+            description = description,
+            name = name,
+            email = email,
+            signature = signature,
+            signatureUse = signatureUse,
+            replyTo = replyTo,
+        )
     }
 }
 
diff --git a/legacy/core/src/test/java/com/fsck/k9/message/MessageBuilderTest.java b/legacy/core/src/test/java/com/fsck/k9/message/MessageBuilderTest.java
index 05da7c56af7..43b8b24ab44 100644
--- a/legacy/core/src/test/java/com/fsck/k9/message/MessageBuilderTest.java
+++ b/legacy/core/src/test/java/com/fsck/k9/message/MessageBuilderTest.java
@@ -523,6 +523,7 @@ private Identity createIdentity() {
                 TEST_IDENTITY_ADDRESS.getAddress(),
                 null,
                 false,
+                false,
                 null
         );
     }
diff --git a/legacy/core/src/test/java/com/fsck/k9/message/TextBodyBuilderHtmlSignatureTest.kt b/legacy/core/src/test/java/com/fsck/k9/message/TextBodyBuilderHtmlSignatureTest.kt
new file mode 100644
index 00000000000..ebc5b6c0aa7
--- /dev/null
+++ b/legacy/core/src/test/java/com/fsck/k9/message/TextBodyBuilderHtmlSignatureTest.kt
@@ -0,0 +1,98 @@
+package com.fsck.k9.message
+
+import assertk.assertThat
+import assertk.assertions.contains
+import assertk.assertions.doesNotContain
+import com.fsck.k9.notification.FakePlatformConfigProvider
+import net.thunderbird.core.logging.legacy.Log
+import net.thunderbird.core.logging.testing.TestLogger
+import net.thunderbird.core.preference.GeneralSettings
+import org.junit.Before
+import org.junit.Test
+import org.mockito.kotlin.doReturn
+import org.mockito.kotlin.mock
+
+/**
+ * Exercises the HTML signature short-circuit added for the HTML signature feature.
+ *
+ * The existing parameterized [TextBodyBuilderTest] covers the plain-text signature
+ * path; these cases add coverage for [TextBodyBuilder.setSignatureIsHtml].
+ */
+class TextBodyBuilderHtmlSignatureTest {
+
+    @Before
+    fun setUp() {
+        Log.logger = TestLogger()
+    }
+
+    private fun newBuilder(messageContent: String = "hello"): TextBodyBuilder {
+        return TextBodyBuilder(
+            messageContent,
+            mock { on { getConfig() } doReturn GeneralSettings(platformConfigProvider = FakePlatformConfigProvider()) },
+        ).apply {
+            setAppendSignature(true)
+            setIncludeQuotedText(false)
+        }
+    }
+
+    @Test
+    fun `html signature is embedded verbatim in html body without text-to-html conversion`() {
+        val htmlSignature = """<p>Sent from <b>Thunderbird</b></p>"""
+        val builder = newBuilder().apply {
+            setSignatureIsHtml(true)
+            setSignature(htmlSignature)
+        }
+
+        val body = builder.buildTextHtml().rawText
+
+        // The <b> tag is preserved — it would have been escaped to &lt;b&gt; if we had
+        // gone through HtmlConverter.textToHtmlFragment().
+        assertThat(body).contains("<b>Thunderbird</b>")
+        assertThat(body).doesNotContain("&lt;b&gt;")
+    }
+
+    @Test
+    fun `html signature has script tags removed when embedded in html body`() {
+        val htmlSignature = """<p>Hi</p><script>alert('xss')</script>"""
+        val builder = newBuilder().apply {
+            setSignatureIsHtml(true)
+            setSignature(htmlSignature)
+        }
+
+        val body = builder.buildTextHtml().rawText
+
+        assertThat(body).contains("<p>Hi</p>")
+        assertThat(body).doesNotContain("<script>")
+        assertThat(body).doesNotContain("alert")
+    }
+
+    @Test
+    fun `html signature is converted to plain text when building plain body`() {
+        val htmlSignature = """<p>Sent from <b>Thunderbird</b></p>"""
+        val builder = newBuilder().apply {
+            setSignatureIsHtml(true)
+            setSignature(htmlSignature)
+        }
+
+        val body = builder.buildTextPlain().rawText
+
+        // The HTML tags should be stripped for the plain-text path.
+        assertThat(body).contains("Sent from Thunderbird")
+        assertThat(body).doesNotContain("<b>")
+        assertThat(body).doesNotContain("<p>")
+    }
+
+    @Test
+    fun `plain signature still goes through text-to-html conversion in html body`() {
+        val plainSignature = "-- \r\nAlice"
+        val builder = newBuilder().apply {
+            setSignatureIsHtml(false)
+            setSignature(plainSignature)
+        }
+
+        val body = builder.buildTextHtml().rawText
+
+        // The plain-text path wraps the signature in a k9mail-signature div.
+        assertThat(body).contains("k9mail-signature")
+    }
+}
diff --git a/legacy/core/src/test/java/com/fsck/k9/message/html/HtmlSignatureSanitizerTest.kt b/legacy/core/src/test/java/com/fsck/k9/message/html/HtmlSignatureSanitizerTest.kt
new file mode 100644
index 00000000000..493fec2f18e
--- /dev/null
+++ b/legacy/core/src/test/java/com/fsck/k9/message/html/HtmlSignatureSanitizerTest.kt
@@ -0,0 +1,79 @@
+package com.fsck.k9.message.html
+
+import assertk.assertThat
+import assertk.assertions.contains
+import assertk.assertions.doesNotContain
+import assertk.assertions.isEmpty
+import assertk.assertions.isEqualTo
+import org.junit.Test
+
+class HtmlSignatureSanitizerTest {
+
+    @Test
+    fun `empty input returns empty string`() {
+        assertThat(HtmlSignatureSanitizer.sanitize("")).isEmpty()
+    }
+
+    @Test
+    fun `preserves basic formatting tags`() {
+        val input = "<p>Hello <b>world</b></p>"
+        assertThat(HtmlSignatureSanitizer.sanitize(input)).contains("<b>world</b>")
+    }
+
+    @Test
+    fun `preserves anchors with https urls`() {
+        val input = """<a href="https://example.com">example</a>"""
+        val result = HtmlSignatureSanitizer.sanitize(input)
+        assertThat(result).contains("""href="https://example.com"""")
+        assertThat(result).contains(">example</a>")
+    }
+
+    @Test
+    fun `preserves img with https src`() {
+        val input = """<img src="https://example.com/logo.png" alt="logo">"""
+        val result = HtmlSignatureSanitizer.sanitize(input)
+        assertThat(result).contains("""src="https://example.com/logo.png"""")
+    }
+
+    @Test
+    fun `preserves inline style attributes`() {
+        val input = """<span style="color: red;">red</span>"""
+        assertThat(HtmlSignatureSanitizer.sanitize(input)).contains("""style="color: red;"""")
+    }
+
+    @Test
+    fun `strips script elements`() {
+        val input = "<p>Hi</p><script>alert('xss')</script>"
+        val result = HtmlSignatureSanitizer.sanitize(input)
+        assertThat(result).doesNotContain("script")
+        assertThat(result).doesNotContain("alert")
+    }
+
+    @Test
+    fun `strips inline event handler attributes`() {
+        val input = """<a href="https://example.com" onclick="alert(1)">click</a>"""
+        val result = HtmlSignatureSanitizer.sanitize(input)
+        assertThat(result).doesNotContain("onclick")
+        assertThat(result).doesNotContain("alert")
+    }
+
+    @Test
+    fun `strips javascript urls from anchors`() {
+        val input = """<a href="javascript:alert(1)">click</a>"""
+        val result = HtmlSignatureSanitizer.sanitize(input)
+        assertThat(result).doesNotContain("javascript")
+        assertThat(result).doesNotContain("alert")
+    }
+
+    @Test
+    fun `strips iframe elements`() {
+        val input = """<iframe src="https://evil.example"></iframe>"""
+        assertThat(HtmlSignatureSanitizer.sanitize(input)).doesNotContain("iframe")
+    }
+
+    @Test
+    fun `plain text passes through unchanged`() {
+        val input = "Just some text"
+        assertThat(HtmlSignatureSanitizer.sanitize(input)).isEqualTo("Just some text")
+    }
+}
diff --git a/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/EditIdentity.kt b/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/EditIdentity.kt
index 1bc6912efd8..6858c4f1429 100644
--- a/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/EditIdentity.kt
+++ b/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/EditIdentity.kt
@@ -30,6 +30,7 @@ class EditIdentity : BaseActivity() {
     private lateinit var replyTo: EditText
     private lateinit var signatureUse: MaterialCheckBox
     private lateinit var signature: EditText
+    private lateinit var signatureIsHtml: MaterialCheckBox
     private lateinit var signatureLayout: View
 
     private var identityIndex: Int = 0
@@ -65,6 +66,7 @@ class EditIdentity : BaseActivity() {
         replyTo = findViewById(R.id.reply_to)
         signatureUse = findViewById(R.id.signature_use)
         signature = findViewById(R.id.signature)
+        signatureIsHtml = findViewById(R.id.signature_is_html)
         signatureLayout = findViewById(R.id.signature_layout)
 
         description.setText(identity.description)
@@ -88,6 +90,8 @@ class EditIdentity : BaseActivity() {
             signatureLayout.isVisible = false
         }
 
+        signatureIsHtml.isChecked = identity.signatureIsHtml
+
         setTextChangedListeners()
         validateFields()
     }
@@ -119,6 +123,7 @@ class EditIdentity : BaseActivity() {
             name = name.text.toString().takeUnless { it.isBlank() },
             signatureUse = signatureUse.isChecked,
             signature = signature.text.toString(),
+            signatureIsHtml = signatureIsHtml.isChecked,
             replyTo = replyTo.text.toString().trim().takeUnless { it.isBlank() },
         )
 
diff --git a/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/MessageCompose.java b/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/MessageCompose.java
index bc57aaeaf29..bfc9a0ccf9c 100644
--- a/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/MessageCompose.java
+++ b/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/MessageCompose.java
@@ -102,6 +102,7 @@
 import com.fsck.k9.helper.Utility;
 import net.thunderbird.core.android.network.ConnectivityManager;
 import net.thunderbird.core.common.mail.Flag;
+import com.fsck.k9.mail.Address;
 import com.fsck.k9.mail.Message;
 import com.fsck.k9.mail.Message.RecipientType;
 import net.thunderbird.core.common.exception.MessagingException;
@@ -226,6 +227,8 @@ public class MessageCompose extends BaseActivity implements OnClickListener,
     private final MessagingController messagingController = DI.get(MessagingController.class);
     private final Preferences preferences = DI.get(Preferences.class);
     private final GeneralSettingsManager generalSettingsManager = DI.get(GeneralSettingsManager.class);
+    private final com.fsck.k9.view.WebViewConfigProvider webViewConfigProvider =
+            DI.get(com.fsck.k9.view.WebViewConfigProvider.class);
 
     private final IntentDataMapper indentDataMapper = DI.get(IntentDataMapper.class);
 
@@ -284,6 +287,7 @@ public class MessageCompose extends BaseActivity implements OnClickListener,
     private MaterialTextView chooseIdentityView;
     private EditText subjectView;
     private EditText signatureView;
+    private com.fsck.k9.view.MessageWebView signatureHtmlPreview;
     private EditText messageContentView;
     private LinearLayout attachmentsView;
 
@@ -372,6 +376,8 @@ public void onCreate(Bundle savedInstanceState) {
 
         EditText upperSignature = findViewById(R.id.upper_signature);
         EditText lowerSignature = findViewById(R.id.lower_signature);
+        com.fsck.k9.view.MessageWebView upperSignaturePreview = findViewById(R.id.upper_signature_html_preview);
+        com.fsck.k9.view.MessageWebView lowerSignaturePreview = findViewById(R.id.lower_signature_html_preview);
 
 
         QuotedMessageMvpView quotedMessageMvpView = new QuotedMessageMvpView(this);
@@ -493,16 +499,34 @@ public void onTextChanged(CharSequence s, int start, int before, int count) {
 
         if (account.isSignatureBeforeQuotedText()) {
             signatureView = upperSignature;
+            signatureHtmlPreview = upperSignaturePreview;
             lowerSignature.setVisibility(View.GONE);
+            lowerSignaturePreview.setVisibility(View.GONE);
         } else {
             signatureView = lowerSignature;
+            signatureHtmlPreview = lowerSignaturePreview;
             upperSignature.setVisibility(View.GONE);
-        }
+            upperSignaturePreview.setVisibility(View.GONE);
+        }
+        signatureHtmlPreview.configure(webViewConfigProvider.createForMessageCompose());
+        // Override MessageWebView's inbound-mail defaults: the signature is the user's own
+        // content, so allow remote images, and render at natural device size rather than
+        // the 980px "wide viewport" that would shrink short signatures.
+        signatureHtmlPreview.blockNetworkData(false);
+        signatureHtmlPreview.getSettings().setUseWideViewPort(false);
+        signatureHtmlPreview.getSettings().setLoadWithOverviewMode(false);
+        signatureHtmlPreview.setWebViewClient(new android.webkit.WebViewClient() {
+            @Override
+            public boolean shouldOverrideUrlLoading(android.webkit.WebView view, String url) {
+                return true;
+            }
+        });
         updateSignature();
         signatureView.addTextChangedListener(signTextWatcher);
 
         if (!identity.getSignatureUse()) {
             signatureView.setVisibility(View.GONE);
+            signatureHtmlPreview.setVisibility(View.GONE);
         }
 
         requestReadReceipt = account.isMessageReadReceipt();
@@ -1087,9 +1111,25 @@ private void updateSignature() {
         if (identity.getSignatureUse()) {
             String signature = CrLfConverter.toLf(identity.getSignature());
             signatureView.setText(signature);
-            signatureView.setVisibility(View.VISIBLE);
+            // The plain EditText can't render HTML, so for HTML signatures we hide it and
+            // show a rendered preview in a WebView instead. The EditText still holds the
+            // raw HTML so signatureView.getText() continues to feed the outgoing message.
+            if (identity.getSignatureIsHtml() && signature != null) {
+                signatureView.setVisibility(View.GONE);
+                String sanitized = com.fsck.k9.message.html.HtmlSignatureSanitizer.sanitize(signature);
+                String document = "<!DOCTYPE html><html><head>" +
+                        "<meta name=\"viewport\" content=\"width=device-width, initial-scale=1\">" +
+                        "<style>body{margin:12px;word-wrap:break-word;}img{max-width:100%;height:auto;}</style>" +
+                        "</head><body>" + sanitized + "</body></html>";
+                signatureHtmlPreview.displayHtmlContentWithInlineAttachments(document, null, null);
+                signatureHtmlPreview.setVisibility(View.VISIBLE);
+            } else {
+                signatureHtmlPreview.setVisibility(View.GONE);
+                signatureView.setVisibility(View.VISIBLE);
+            }
         } else {
             signatureView.setVisibility(View.GONE);
+            signatureHtmlPreview.setVisibility(View.GONE);
         }
     }
 
@@ -1569,6 +1609,27 @@ private void processDraftMessage(MessageViewInfo messageViewInfo) {
             newIdentity = newIdentity.withEmail(identity.getEmail());
         }
 
+        // The draft's identity header does not encode whether the signature is HTML, so
+        // inherit the flag from the matching account identity (looked up by email) so that
+        // multi-identity accounts preserve each identity's setting. If no match, fall back
+        // to the currently-loaded default identity.
+        Identity matchedIdentity = null;
+        String draftEmail = newIdentity.getEmail();
+        if (draftEmail != null) {
+            try {
+                Address[] parsed = Address.parse(draftEmail);
+                if (parsed.length > 0) {
+                    matchedIdentity = account.findIdentity(parsed[0]);
+                }
+            } catch (Exception e) {
+                // Ignore — fall back to default identity below.
+            }
+        }
+        boolean signatureIsHtml = matchedIdentity != null
+                ? matchedIdentity.getSignatureIsHtml()
+                : identity.getSignatureIsHtml();
+        newIdentity = newIdentity.withSignatureIsHtml(signatureIsHtml);
+
         if (k9identity.containsKey(IdentityField.ORIGINAL_MESSAGE)) {
             relatedMessageReference = null;
             String originalMessage = k9identity.get(IdentityField.ORIGINAL_MESSAGE);
diff --git a/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/setup/AccountSetupComposition.kt b/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/setup/AccountSetupComposition.kt
index 1cfaa92797f..bb62b2138d2 100644
--- a/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/setup/AccountSetupComposition.kt
+++ b/legacy/ui/legacy/src/main/java/com/fsck/k9/activity/setup/AccountSetupComposition.kt
@@ -28,6 +28,7 @@ class AccountSetupComposition : BaseActivity() {
     private lateinit var accountAlwaysBcc: EditText
     private lateinit var accountSenderName: EditText
     private lateinit var accountSignatureUse: MaterialCheckBox
+    private lateinit var accountSignatureIsHtml: MaterialCheckBox
     private lateinit var accountSignatureBeforeLocation: MaterialRadioButton
     private lateinit var accountSignatureAfterLocation: MaterialRadioButton
     private lateinit var accountSignatureLayout: LinearLayout
@@ -49,6 +50,7 @@ class AccountSetupComposition : BaseActivity() {
         accountSignatureLayout = findViewById(R.id.account_signature_layout)
         accountSignatureUse = findViewById(R.id.account_signature_use)
         accountSignature = findViewById(R.id.account_signature)
+        accountSignatureIsHtml = findViewById(R.id.account_signature_is_html)
         accountSignatureBeforeLocation = findViewById(R.id.account_signature_location_before_quoted_text)
         accountSignatureAfterLocation = findViewById(R.id.account_signature_location_after_quoted_text)
 
@@ -62,6 +64,7 @@ class AccountSetupComposition : BaseActivity() {
             if (isChecked) {
                 accountSignatureLayout.isVisible = true
                 accountSignature.setText(account.signature)
+                accountSignatureIsHtml.isChecked = account.signatureIsHtml
 
                 val isSignatureBeforeQuotedText = account.isSignatureBeforeQuotedText
                 accountSignatureBeforeLocation.isChecked = isSignatureBeforeQuotedText
@@ -73,6 +76,7 @@ class AccountSetupComposition : BaseActivity() {
 
         if (useSignature) {
             accountSignature.setText(account.signature)
+            accountSignatureIsHtml.isChecked = account.signatureIsHtml
 
             val isSignatureBeforeQuotedText = account.isSignatureBeforeQuotedText
             accountSignatureBeforeLocation.setChecked(isSignatureBeforeQuotedText)
@@ -131,6 +135,7 @@ class AccountSetupComposition : BaseActivity() {
         account.signatureUse = accountSignatureUse.isChecked
         if (accountSignatureUse.isChecked) {
             account.signature = accountSignature.text.toString()
+            account.signatureIsHtml = accountSignatureIsHtml.isChecked
             account.isSignatureBeforeQuotedText = accountSignatureBeforeLocation.isChecked
         }
 
diff --git a/legacy/ui/legacy/src/main/res/layout/account_setup_composition.xml b/legacy/ui/legacy/src/main/res/layout/account_setup_composition.xml
index 653d66031e8..3f3f762cdf8 100644
--- a/legacy/ui/legacy/src/main/res/layout/account_setup_composition.xml
+++ b/legacy/ui/legacy/src/main/res/layout/account_setup_composition.xml
@@ -113,6 +113,20 @@
                     android:inputType="textMultiLine|textAutoCorrect|textCapSentences"
                     />
 
+                <com.google.android.material.checkbox.MaterialCheckBox
+                    android:id="@+id/account_signature_is_html"
+                    android:layout_width="fill_parent"
+                    android:layout_height="wrap_content"
+                    android:text="@string/account_settings_signature_is_html_label"
+                    />
+
+                <com.google.android.material.textview.MaterialTextView
+                    android:layout_width="fill_parent"
+                    android:layout_height="wrap_content"
+                    android:text="@string/account_settings_signature_is_html_summary"
+                    android:textAppearance="?attr/textAppearanceBodySmall"
+                    />
+
                 <com.google.android.material.textview.MaterialTextView
                     android:layout_width="fill_parent"
                     android:layout_height="wrap_content"
diff --git a/legacy/ui/legacy/src/main/res/layout/edit_identity.xml b/legacy/ui/legacy/src/main/res/layout/edit_identity.xml
index aed59a8fe2c..ecd03c96c73 100644
--- a/legacy/ui/legacy/src/main/res/layout/edit_identity.xml
+++ b/legacy/ui/legacy/src/main/res/layout/edit_identity.xml
@@ -137,6 +137,21 @@
                     android:layout_width="fill_parent"
                     android:hint="@string/edit_identity_signature_hint"
                     />
+
+                <com.google.android.material.checkbox.MaterialCheckBox
+                    android:id="@+id/signature_is_html"
+                    android:layout_width="fill_parent"
+                    android:layout_height="wrap_content"
+                    android:text="@string/account_settings_signature_is_html_label"
+                    />
+
+                <com.google.android.material.textview.MaterialTextView
+                    android:id="@+id/signature_is_html_summary"
+                    android:layout_width="fill_parent"
+                    android:layout_height="wrap_content"
+                    android:text="@string/account_settings_signature_is_html_summary"
+                    android:textAppearance="?attr/textAppearanceBodySmall"
+                    />
             </LinearLayout>
         </LinearLayout>
     </ScrollView>
diff --git a/legacy/ui/legacy/src/main/res/layout/message_compose_content.xml b/legacy/ui/legacy/src/main/res/layout/message_compose_content.xml
index 4dd8b1835ac..f340f902895 100644
--- a/legacy/ui/legacy/src/main/res/layout/message_compose_content.xml
+++ b/legacy/ui/legacy/src/main/res/layout/message_compose_content.xml
@@ -84,6 +84,13 @@ the 'composer theme' setting).
             android:textAppearance="?attr/textAppearanceBodyLarge"
             />
 
+        <com.fsck.k9.view.MessageWebView
+            android:id="@+id/upper_signature_html_preview"
+            android:layout_width="fill_parent"
+            android:layout_height="wrap_content"
+            android:visibility="gone"
+            />
+
         <com.google.android.material.button.MaterialButton
             android:id="@+id/quoted_text_show"
             android:text="@string/message_compose_show_quoted_text_action"
@@ -160,6 +167,13 @@ the 'composer theme' setting).
             android:textAppearance="?attr/textAppearanceBodyLarge"
             />
 
+        <com.fsck.k9.view.MessageWebView
+            android:id="@+id/lower_signature_html_preview"
+            android:layout_width="fill_parent"
+            android:layout_height="wrap_content"
+            android:visibility="gone"
+            />
+
         <!--
             Empty container for storing attachments. We'll stick
             instances of message_compose_attachment.xml in here.
diff --git a/legacy/ui/legacy/src/main/res/values/strings.xml b/legacy/ui/legacy/src/main/res/values/strings.xml
index 623d4454eed..8024ce5ef74 100644
--- a/legacy/ui/legacy/src/main/res/values/strings.xml
+++ b/legacy/ui/legacy/src/main/res/values/strings.xml
@@ -619,6 +619,8 @@
 
     <string name="account_settings_signature_use_label">Use Signature</string>
     <string name="account_settings_signature_label">Signature</string>
+    <string name="account_settings_signature_is_html_label">Format signature as HTML</string>
+    <string name="account_settings_signature_is_html_summary">Interpret the signature as HTML. Scripts and event handlers will be removed for security.</string>
 
     <string name="default_identity_description">Initial identity</string>
     <string name="choose_identity_title">Choose identity</string>
diff --git a/legacy/ui/legacy/src/test/java/com/fsck/k9/activity/compose/ReplyToPresenterTest.kt b/legacy/ui/legacy/src/test/java/com/fsck/k9/activity/compose/ReplyToPresenterTest.kt
index 557860609eb..38ef8db1bfb 100644
--- a/legacy/ui/legacy/src/test/java/com/fsck/k9/activity/compose/ReplyToPresenterTest.kt
+++ b/legacy/ui/legacy/src/test/java/com/fsck/k9/activity/compose/ReplyToPresenterTest.kt
@@ -74,7 +74,14 @@ class ReplyToPresenterTest : RobolectricTest() {
 
     @Test
     fun testSetIdentity_identityWithOneReplyTo_expectSetReplyTo() {
-        val identity = Identity("a", "b", "x@y.z", null, false, REPLY_TO_ADDRESS)
+        val identity = Identity(
+            description = "a",
+            name = "b",
+            email = "x@y.z",
+            signature = null,
+            signatureUse = false,
+            replyTo = REPLY_TO_ADDRESS,
+        )
 
         replyToPresenter.setIdentity(identity)
 
@@ -84,7 +91,14 @@ class ReplyToPresenterTest : RobolectricTest() {
     @Test
     fun testSetIdentity_identityWithMultipleReplyTo_expectSetReplyTo() {
         val replyTo = "$REPLY_TO_ADDRESS, $REPLY_TO_ADDRESS_2"
-        val identity = Identity("a", "b", "x@y.z", null, false, replyTo)
+        val identity = Identity(
+            description = "a",
+            name = "b",
+            email = "x@y.z",
+            signature = null,
+            signatureUse = false,
+            replyTo = replyTo,
+        )
 
         replyToPresenter.setIdentity(identity)
 
@@ -94,7 +108,14 @@ class ReplyToPresenterTest : RobolectricTest() {
     @Test
     fun testOnSwitchIdentity_newIdentityWithoutReplyTo_expectRemoveReplyToOfOldIdentity() {
         val replyToOne = "$REPLY_TO_ADDRESS, $REPLY_TO_ADDRESS_2"
-        val identityOne = Identity("a", "b", "x@y.z", null, false, replyToOne)
+        val identityOne = Identity(
+            description = "a",
+            name = "b",
+            email = "x@y.z",
+            signature = null,
+            signatureUse = false,
+            replyTo = replyToOne,
+        )
         val identityTwo = Identity()
 
         replyToPresenter.setIdentity(identityOne)
@@ -106,9 +127,23 @@ class ReplyToPresenterTest : RobolectricTest() {
     @Test
     fun testOnSwitchIdentity_identityWithSubsetOfOldIdentity_expectRemoveThenAdd() {
         val replyToOne = "$REPLY_TO_ADDRESS, $REPLY_TO_ADDRESS_2"
-        val identityOne = Identity("a", "b", "x@y.z", null, false, replyToOne)
+        val identityOne = Identity(
+            description = "a",
+            name = "b",
+            email = "x@y.z",
+            signature = null,
+            signatureUse = false,
+            replyTo = replyToOne,
+        )
         val replyToTwo = "$REPLY_TO_ADDRESS, $REPLY_TO_ADDRESS_3"
-        val identityTwo = Identity("c", "d", "x@y.z", null, false, replyToTwo)
+        val identityTwo = Identity(
+            description = "c",
+            name = "d",
+            email = "x@y.z",
+            signature = null,
+            signatureUse = false,
+            replyTo = replyToTwo,
+        )
 
         replyToPresenter.setIdentity(identityOne)
         replyToPresenter.setIdentity(identityTwo)