Support SmtpAuthenticationMode.Ntlm

Author: snakefoot

src/NLog.MailKit/MailTarget.cs

@@ -35,6 +35,7 @@
 using System.Collections.Generic;
 using System.ComponentModel;
 using System.Linq;
+using System.Net;
 using System.Text;
 using MailKit.Net.Smtp;
 using MailKit.Security;
@@ -404,7 +405,7 @@ private void SendMailMessage(MimeMessage message, LogEventInfo lastEvent)
                 var skipCertificateValidation = RenderLogEvent(SkipCertificateValidation, lastEvent);
                 if (skipCertificateValidation)
                 {
-                    client.ServerCertificateValidationCallback += (s, cert, chain, sslPolicyErrors) => true;
+                    client.ServerCertificateValidationCallback += static (s, cert, chain, sslPolicyErrors) => true;
                 }
 
                 if (requireTLS)
@@ -446,6 +447,10 @@ private void SendMailMessage(MimeMessage message, LogEventInfo lastEvent)
                     var oauth2 = new SaslMechanismOAuth2(userName, oauth2Token);
                     client.Authenticate(oauth2);
                 }
+                else if (smtpAuthentication == SmtpAuthenticationMode.Ntlm)
+                {
+                    client.Authenticate(new SaslMechanismNtlm(CredentialCache.DefaultNetworkCredentials));
+                }
 
                 client.Send(message);
                 InternalLogger.Trace("{0}: Sending mail done. Disconnecting", this);