fix: normalize folded headers before SMTP send

src/main/java/io/vertx/ext/mail/impl/SMTPSendMail.java

@@ -287,7 +287,7 @@ private Future<MailResult> sendMailData(boolean includeData) {
 
   private Future<Void> sendMailHeaders(MultiMap headers) {
     StringBuilder sb = new StringBuilder();
-    headers.forEach(header -> sb.append(header.getKey()).append(": ").append(header.getValue()).append("\r\n"));
+    headers.forEach(header -> sb.append(header.getKey()).append(": ").append(Utils.normalizeSmtpLineBreaks(header.getValue())).append("\r\n"));
     final String headerLines = sb.toString();
     return connection.writeLineWithDrain(headerLines, written.getAndAdd(headerLines.length()) < 1000);
   }

src/main/java/io/vertx/ext/mail/impl/Utils.java

@@ -129,4 +129,18 @@ public static <T> List<T> asList(T element) {
     return Collections.singletonList(element);
   }
 
+  /**
+   * Normalizes embedded header line breaks to CRLF so folded headers are sent
+   * on the wire with RFC-compliant separators.
+   *
+   * @param value the header value
+   * @return the normalized header value
+   */
+  public static String normalizeSmtpLineBreaks(String value) {
+    if (value == null || value.indexOf('\n') == -1 && value.indexOf('\r') == -1) {
+      return value;
+    }
+    return value.replace("\r\n", "\n").replace('\r', '\n').replace("\n", "\r\n");
+  }
+
 }

src/main/java/io/vertx/ext/mail/impl/dkim/DKIMSigner.java

@@ -16,6 +16,8 @@
 
 package io.vertx.ext.mail.impl.dkim;
 
+import static io.vertx.ext.mail.impl.Utils.normalizeSmtpLineBreaks;
+
 import io.vertx.codegen.annotations.Nullable;
 import io.vertx.core.*;
 import io.vertx.core.buffer.Buffer;
@@ -414,7 +416,7 @@ private String copiedHeaders(List<String> headers, EncodedPart encodedMessage) {
     return headers.stream().map(h -> {
       String hValue = encodedMessage.headers().get(h);
       if (hValue != null) {
-        return h + ":" + dkimQuotedPrintableCopiedHeader(hValue);
+        return h + ":" + dkimQuotedPrintableCopiedHeader(normalizeSmtpLineBreaks(hValue));
       }
       throw new RuntimeException("Unknown email header: " + h + " in copied headers.");
     }).collect(Collectors.joining("|"));
@@ -444,11 +446,12 @@ private String dkimQuotedPrintableCopiedHeader(String value) {
    * @return the canonicalization email header in format of 'Name':'Value'.
    */
   public String canonicHeader(String emailHeaderName, String emailHeaderValue) {
+    String normalizedHeaderValue = normalizeSmtpLineBreaks(emailHeaderValue);
     if (this.dkimSignOptions.getHeaderCanonAlgo() == CanonicalizationAlgorithm.SIMPLE) {
-      return emailHeaderName + ": " + emailHeaderValue;
+      return emailHeaderName + ": " + normalizedHeaderValue;
     }
     String headerName = emailHeaderName.trim().toLowerCase();
-    return headerName + ":" + canonicalLine(emailHeaderValue, this.dkimSignOptions.getHeaderCanonAlgo());
+    return headerName + ":" + canonicalLine(normalizedHeaderValue, this.dkimSignOptions.getHeaderCanonAlgo());
   }
 
   public String dkimMailBody(String mailBody) {

src/test/java/io/vertx/tests/mail/client/MailHeaderLineBreakTest.java

@@ -0,0 +1,37 @@
+/*
+ *  Copyright (c) 2011-2026 The original author or authors
+ *
+ *  All rights reserved. This program and the accompanying materials
+ *  are made available under the terms of the Eclipse Public License v1.0
+ *  and Apache License v2.0 which accompanies this distribution.
+ *
+ *       The Eclipse Public License is available at
+ *       http://www.eclipse.org/legal/epl-v10.html
+ *
+ *       The Apache License v2.0 is available at
+ *       http://www.opensource.org/licenses/apache2.0.php
+ *
+ *  You may elect to redistribute this code under either of these licenses.
+ */
+
+package io.vertx.tests.mail.client;
+
+import io.vertx.ext.mail.MailClient;
+import io.vertx.ext.mail.MailMessage;
+import io.vertx.ext.unit.TestContext;
+import io.vertx.ext.unit.junit.VertxUnitRunner;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+@RunWith(VertxUnitRunner.class)
+public class MailHeaderLineBreakTest extends SMTPTestWiser {
+
+  @Test
+  public void testFoldedSubjectUsesCrLf(TestContext testContext) {
+    this.testContext = testContext;
+    MailClient mailClient = mailClientLogin();
+
+    MailMessage message = exampleMessage().setSubject("Unicode folding test ⭐⭐⭐⭐⭐⭐⭐⭐⭐⭐⭐⭐⭐⭐⭐⭐");
+    testSuccess(mailClient, message);
+  }
+}

src/test/java/io/vertx/tests/mail/internal/dkim/DKIMSignerTest.java

@@ -172,7 +172,7 @@ public void testDKIMHeaderTemplate() {
   public void testSimpleHeader() {
     DKIMSignOptions dkimOps = dkimOps().setHeaderCanonAlgo(CanonicalizationAlgorithm.SIMPLE);
     DKIMSigner signer = new DKIMSigner(dkimOps, null);
-    // there will be possible to have \n in the header value, keep it as it is.
+    // folded headers are normalized to CRLF before signing.
     String name = "from";
     String value = "user@example.com";
     String canonicHeaderLine = signer.canonicHeader(name, value);
@@ -191,14 +191,14 @@ public void testSimpleHeader() {
     name = " from ";
     value = " user@example.com \n ";
     canonicHeaderLine = signer.canonicHeader(name, value);
-    assertEquals(" from :  user@example.com \n ", canonicHeaderLine);
+    assertEquals(" from :  user@example.com \r\n ", canonicHeaderLine);
   }
 
   @Test
   public void testRelaxedHeader() {
     DKIMSignOptions dkimOps = dkimOps().setHeaderCanonAlgo(CanonicalizationAlgorithm.RELAXED);
     DKIMSigner signer = new DKIMSigner(dkimOps, null);
-    // there will be possible to have \n in the header value
+    // folded headers are normalized to CRLF before relaxed canonicalization.
     String name = "From";
     String value = "user@example.com";
     String canonicHeaderLine = signer.canonicHeader(name, value);