diff --git a/src/Senders/FluentEmail.MailKit/MailKitSender.cs b/src/Senders/FluentEmail.MailKit/MailKitSender.cs
index 8a5d1413..f1ab2a2e 100644
--- a/src/Senders/FluentEmail.MailKit/MailKitSender.cs
+++ b/src/Senders/FluentEmail.MailKit/MailKitSender.cs
@@ -181,11 +181,13 @@ private async Task SaveToPickupDirectory(MimeMessage message, string pickupDirec
         }
 
         /// <summary>
-        /// Create a MimMessage so MailKit can send it
+        /// Create a MimeMessage so MailKit can send it.
+        /// Virtual so subclasses can preprocess the message before sending
+        /// (e.g. DKIM-sign it) — see https://github.com/lukencode/FluentEmail/issues/388
         /// </summary>
         /// <returns>The mail message.</returns>
         /// <param name="email">Email data.</param>
-        private MimeMessage CreateMailMessage(IFluentEmail email)
+        protected virtual MimeMessage CreateMailMessage(IFluentEmail email)
         {
             var data = email.Data;
 
diff --git a/test/FluentEmail.Core.Tests/MailKitSmtpSenderTests.cs b/test/FluentEmail.Core.Tests/MailKitSmtpSenderTests.cs
index 7b80921b..2b8f066e 100644
--- a/test/FluentEmail.Core.Tests/MailKitSmtpSenderTests.cs
+++ b/test/FluentEmail.Core.Tests/MailKitSmtpSenderTests.cs
@@ -1,7 +1,10 @@
 using System.IO;
+using System.Linq;
 using System.Threading.Tasks;
 using FluentEmail.Core;
+using FluentEmail.Core.Interfaces;
 using FluentEmail.MailKitSmtp;
+using MimeKit;
 using NUnit.Framework;
 using Attachment = FluentEmail.Core.Models.Attachment;
 
@@ -119,5 +122,50 @@ public void CanSendHtmlAndPlaintextTogether()
 
             Assert.IsTrue(response.Successful);
         }
+
+        [Test]
+        public void CanPreprocessMessageByOverridingCreateMailMessage()
+        {
+            var sender = new PreprocessingMailKitSender(new SmtpClientOptions
+            {
+                Server = "localhost",
+                Port = 25,
+                UseSsl = false,
+                RequiresAuthentication = false,
+                UsePickupDirectory = true,
+                MailPickupDirectory = tempDirectory
+            });
+            Email.DefaultSender = sender;
+
+            var email = Email
+                .From(fromEmail)
+                .To(toEmail)
+                .Subject(subject)
+                .Body(body);
+
+            var response = email.Send();
+
+            Assert.IsTrue(response.Successful);
+            var eml = Directory.EnumerateFiles(tempDirectory, "*.eml").First();
+            var content = File.ReadAllText(eml);
+            Assert.IsTrue(content.Contains("X-Preprocessed: yes"),
+                "Override of CreateMailMessage should be able to modify the message before sending.");
+        }
+
+        /// <summary>
+        /// Simulates e.g. DKIM-signing: preprocess the MimeMessage after creation.
+        /// Possible since CreateMailMessage is protected virtual (issue #388).
+        /// </summary>
+        private class PreprocessingMailKitSender : MailKitSender
+        {
+            public PreprocessingMailKitSender(SmtpClientOptions options) : base(options) { }
+
+            protected override MimeMessage CreateMailMessage(IFluentEmail email)
+            {
+                var message = base.CreateMailMessage(email);
+                message.Headers.Add("X-Preprocessed", "yes");
+                return message;
+            }
+        }
     }
 }