diff --git a/Wino.Core.Domain/Extensions/MailHeaderExtensions.cs b/Wino.Core.Domain/Extensions/MailHeaderExtensions.cs
index aa41729b..2b686ff7 100644
--- a/Wino.Core.Domain/Extensions/MailHeaderExtensions.cs
+++ b/Wino.Core.Domain/Extensions/MailHeaderExtensions.cs
@@ -1,10 +1,26 @@
 using System;
+using System.Collections.Generic;
 using System.Linq;
 
 namespace Wino.Core.Domain.Extensions;
 
 public static class MailHeaderExtensions
 {
+    public static string NormalizeMessageId(string value)
+    {
+        if (value == null)
+            return null;
+
+        var normalized = StripAngleBrackets(value)?.Trim();
+        return string.IsNullOrWhiteSpace(normalized) ? string.Empty : normalized;
+    }
+
+    public static string ToHeaderMessageId(string value)
+    {
+        var normalized = NormalizeMessageId(value);
+        return string.IsNullOrEmpty(normalized) ? string.Empty : $"<{normalized}>";
+    }
+
     /// <summary>
     /// Strips angle brackets from a Message-ID or In-Reply-To value.
     /// RFC 5322 Message-IDs are formatted as &lt;id@domain&gt;, but MimeKit
@@ -29,14 +45,53 @@ public static class MailHeaderExtensions
     /// like "&lt;id1@domain&gt; &lt;id2@domain&gt;". This converts them to "id1@domain;id2@domain".
     /// </summary>
     public static string NormalizeReferences(string rawReferences)
+        => JoinStoredReferences(SplitMessageIds(rawReferences));
+
+    public static IEnumerable<string> SplitMessageIds(string values)
     {
-        if (string.IsNullOrEmpty(rawReferences)) return rawReferences;
+        if (string.IsNullOrWhiteSpace(values))
+            return [];
 
-        var ids = rawReferences
+        return values
             .Split(new[] { ' ', '\t', '\r', '\n', ';', ',' }, StringSplitOptions.RemoveEmptyEntries)
-            .Select(StripAngleBrackets)
+            .Select(NormalizeMessageId)
             .Where(id => !string.IsNullOrEmpty(id));
+    }
 
-        return string.Join(";", ids);
+    public static string JoinStoredReferences(IEnumerable<string> values)
+        => string.Join(";", NormalizeDistinctMessageIds(values));
+
+    public static string BuildReferencesHeaderValue(IEnumerable<string> values)
+        => string.Join(" ", NormalizeDistinctMessageIds(values).Select(ToHeaderMessageId));
+
+    public static List<string> BuildReferencesChain(IEnumerable<string> existingReferences, string parentMessageId)
+    {
+        var results = NormalizeDistinctMessageIds(existingReferences).ToList();
+        var normalizedParentMessageId = NormalizeMessageId(parentMessageId);
+
+        if (!string.IsNullOrEmpty(normalizedParentMessageId) &&
+            !results.Contains(normalizedParentMessageId, StringComparer.OrdinalIgnoreCase))
+        {
+            results.Add(normalizedParentMessageId);
+        }
+
+        return results;
+    }
+
+    private static IEnumerable<string> NormalizeDistinctMessageIds(IEnumerable<string> values)
+    {
+        if (values == null)
+            yield break;
+
+        var seen = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+
+        foreach (var value in values)
+        {
+            var normalized = NormalizeMessageId(value);
+            if (string.IsNullOrEmpty(normalized) || !seen.Add(normalized))
+                continue;
+
+            yield return normalized;
+        }
     }
 }
diff --git a/Wino.Core.Domain/Misc/MessageIdGenerator.cs b/Wino.Core.Domain/Misc/MessageIdGenerator.cs
new file mode 100644
index 00000000..36e6d72e
--- /dev/null
+++ b/Wino.Core.Domain/Misc/MessageIdGenerator.cs
@@ -0,0 +1,13 @@
+using System;
+
+namespace Wino.Core.Domain.Misc;
+
+public static class MessageIdGenerator
+{
+    private const string Domain = "wino-mail.app";
+
+    public static string Generate()
+    {
+        return $"<{Guid.NewGuid()}@{Domain}>";
+    }
+}
diff --git a/Wino.Core.Tests/Services/MailHeaderExtensionsTests.cs b/Wino.Core.Tests/Services/MailHeaderExtensionsTests.cs
new file mode 100644
index 00000000..776b93b4
--- /dev/null
+++ b/Wino.Core.Tests/Services/MailHeaderExtensionsTests.cs
@@ -0,0 +1,27 @@
+using FluentAssertions;
+using Wino.Core.Domain.Extensions;
+using Wino.Core.Domain.Misc;
+using Xunit;
+
+namespace Wino.Core.Tests.Services;
+
+public class MailHeaderExtensionsTests
+{
+    [Fact]
+    public void MessageIdGenerator_Generate_ReturnsGuidAtWinoMailDomain()
+    {
+        var generated = MessageIdGenerator.Generate();
+
+        generated.Should().MatchRegex("^<[0-9a-fA-F-]{36}@wino-mail\\.app>$");
+    }
+
+    [Fact]
+    public void BuildReferencesChain_DeduplicatesAndAppendsParentMessageId()
+    {
+        var chain = MailHeaderExtensions.BuildReferencesChain(
+            ["<root@domain.com>", "middle@domain.com", "<middle@domain.com>"],
+            "<parent@domain.com>");
+
+        chain.Should().Equal("root@domain.com", "middle@domain.com", "parent@domain.com");
+    }
+}
diff --git a/Wino.Core.Tests/Services/MailThreadingTests.cs b/Wino.Core.Tests/Services/MailThreadingTests.cs
new file mode 100644
index 00000000..cec68654
--- /dev/null
+++ b/Wino.Core.Tests/Services/MailThreadingTests.cs
@@ -0,0 +1,243 @@
+using FluentAssertions;
+using MimeKit;
+using Moq;
+using Wino.Core.Domain.Entities.Mail;
+using Wino.Core.Domain.Entities.Shared;
+using Wino.Core.Domain.Enums;
+using Wino.Core.Domain.Extensions;
+using Wino.Core.Domain.Interfaces;
+using Wino.Core.Domain.Models.MailItem;
+using Wino.Core.Tests.Helpers;
+using Wino.Services;
+using Xunit;
+
+namespace Wino.Core.Tests.Services;
+
+public class MailThreadingTests : IAsyncLifetime
+{
+    private InMemoryDatabaseService _databaseService = null!;
+    private MailService _mailService = null!;
+    private MailAccount _account = null!;
+    private MailItemFolder _draftFolder = null!;
+
+    public async Task InitializeAsync()
+    {
+        _databaseService = new InMemoryDatabaseService();
+        await _databaseService.InitializeAsync();
+
+        _account = new MailAccount
+        {
+            Id = Guid.NewGuid(),
+            Name = "Threading Test Account",
+            Address = "me@test.local",
+            SenderName = "Test User",
+            ProviderType = MailProviderType.IMAP4
+        };
+
+        _draftFolder = new MailItemFolder
+        {
+            Id = Guid.NewGuid(),
+            MailAccountId = _account.Id,
+            FolderName = "Drafts",
+            SpecialFolderType = SpecialFolderType.Draft,
+            IsSystemFolder = true,
+            IsSynchronizationEnabled = true
+        };
+
+        var preferences = new MailAccountPreferences
+        {
+            Id = Guid.NewGuid(),
+            AccountId = _account.Id,
+            IsNotificationsEnabled = true,
+            IsSignatureEnabled = false
+        };
+
+        var alias = new MailAccountAlias
+        {
+            Id = Guid.NewGuid(),
+            AccountId = _account.Id,
+            AliasAddress = _account.Address,
+            ReplyToAddress = _account.Address,
+            IsPrimary = true,
+            IsRootAlias = true,
+            IsVerified = true
+        };
+
+        await _databaseService.Connection.InsertAsync(_account, typeof(MailAccount));
+        await _databaseService.Connection.InsertAsync(_draftFolder, typeof(MailItemFolder));
+        await _databaseService.Connection.InsertAsync(preferences, typeof(MailAccountPreferences));
+        await _databaseService.Connection.InsertAsync(alias, typeof(MailAccountAlias));
+
+        _mailService = BuildMailService(_databaseService);
+    }
+
+    public async Task DisposeAsync() => await _databaseService.DisposeAsync();
+
+    [Fact]
+    public async Task CreateDraftAsync_EmptyDraft_AssignsGeneratedMessageId()
+    {
+        var (draftMailCopy, draftBase64MimeMessage) = await _mailService.CreateDraftAsync(
+            _account.Id,
+            new DraftCreationOptions { Reason = DraftCreationReason.Empty });
+
+        var mimeMessage = draftBase64MimeMessage.GetMimeMessageFromBase64();
+
+        draftMailCopy.MessageId.Should().MatchRegex("^[0-9a-fA-F-]{36}@wino-mail\\.app$");
+        mimeMessage.MessageId.Should().Be(draftMailCopy.MessageId);
+        mimeMessage.Headers[HeaderId.MessageId].Should().Be(MailHeaderExtensions.ToHeaderMessageId(draftMailCopy.MessageId));
+    }
+
+    [Fact]
+    public async Task CreateDraftAsync_Reply_SetsInReplyToReferencesAndReplySubject()
+    {
+        const string parentMessageId = "original@domain.com";
+
+        var referencedMimeMessage = CreateReferencedMimeMessage("From outlook", parentMessageId);
+        var referencedMailCopy = new MailCopy
+        {
+            UniqueId = Guid.NewGuid(),
+            Id = Guid.NewGuid().ToString(),
+            ThreadId = "provider-thread-id",
+            MessageId = parentMessageId
+        };
+
+        var (draftMailCopy, draftBase64MimeMessage) = await _mailService.CreateDraftAsync(
+            _account.Id,
+            new DraftCreationOptions
+            {
+                Reason = DraftCreationReason.Reply,
+                ReferencedMessage = new ReferencedMessage
+                {
+                    MimeMessage = referencedMimeMessage,
+                    MailCopy = referencedMailCopy
+                }
+            });
+
+        var mimeMessage = draftBase64MimeMessage.GetMimeMessageFromBase64();
+
+        draftMailCopy.InReplyTo.Should().Be(parentMessageId);
+        draftMailCopy.References.Should().Be(parentMessageId);
+        draftMailCopy.Subject.Should().Be("Re: From outlook");
+        draftMailCopy.ThreadId.Should().Be(referencedMailCopy.ThreadId);
+
+        mimeMessage.InReplyTo.Should().Be(parentMessageId);
+        MailHeaderExtensions.NormalizeReferences(mimeMessage.Headers[HeaderId.References]).Should().Be(parentMessageId);
+    }
+
+    [Fact]
+    public async Task CreateDraftAsync_Reply_AppendsReferencesChainOnce()
+    {
+        const string rootMessageId = "root@domain.com";
+        const string middleMessageId = "middle@domain.com";
+        const string parentMessageId = "parent@domain.com";
+
+        var referencedMimeMessage = CreateReferencedMimeMessage("Re: Existing subject", parentMessageId);
+        referencedMimeMessage.References.Add(rootMessageId);
+        referencedMimeMessage.References.Add(middleMessageId);
+
+        var (draftMailCopy, draftBase64MimeMessage) = await _mailService.CreateDraftAsync(
+            _account.Id,
+            new DraftCreationOptions
+            {
+                Reason = DraftCreationReason.Reply,
+                ReferencedMessage = new ReferencedMessage
+                {
+                    MimeMessage = referencedMimeMessage,
+                    MailCopy = new MailCopy { UniqueId = Guid.NewGuid(), Id = Guid.NewGuid().ToString(), MessageId = parentMessageId }
+                }
+            });
+
+        var mimeMessage = draftBase64MimeMessage.GetMimeMessageFromBase64();
+
+        draftMailCopy.References.Should().Be($"{rootMessageId};{middleMessageId};{parentMessageId}");
+        draftMailCopy.Subject.Should().Be("Re: Existing subject");
+        MailHeaderExtensions.NormalizeReferences(mimeMessage.Headers[HeaderId.References])
+            .Should().Be($"{rootMessageId};{middleMessageId};{parentMessageId}");
+    }
+
+    [Fact]
+    public async Task CreateDraftAsync_Reply_FallsBackToReferencedMailCopyThreadingMetadata()
+    {
+        const string rootMessageId = "root@domain.com";
+        const string parentMessageId = "copy-parent@domain.com";
+
+        var referencedMimeMessage = CreateReferencedMimeMessage("Fallback subject");
+        referencedMimeMessage.Headers.Remove(HeaderId.MessageId);
+
+        var referencedMailCopy = new MailCopy
+        {
+            UniqueId = Guid.NewGuid(),
+            Id = Guid.NewGuid().ToString(),
+            MessageId = parentMessageId,
+            References = rootMessageId
+        };
+
+        var (draftMailCopy, _) = await _mailService.CreateDraftAsync(
+            _account.Id,
+            new DraftCreationOptions
+            {
+                Reason = DraftCreationReason.Reply,
+                ReferencedMessage = new ReferencedMessage
+                {
+                    MimeMessage = referencedMimeMessage,
+                    MailCopy = referencedMailCopy
+                }
+            });
+
+        draftMailCopy.InReplyTo.Should().Be(parentMessageId);
+        draftMailCopy.References.Should().Be($"{rootMessageId};{parentMessageId}");
+    }
+
+    private static MimeMessage CreateReferencedMimeMessage(string subject, string? messageId = null)
+    {
+        var message = new MimeMessage();
+        message.From.Add(new MailboxAddress("Sender", "sender@example.com"));
+        message.To.Add(new MailboxAddress("Recipient", "recipient@example.com"));
+        message.Subject = subject;
+        message.Body = new TextPart("plain") { Text = "Body" };
+
+        if (!string.IsNullOrWhiteSpace(messageId))
+            message.MessageId = messageId;
+
+        return message;
+    }
+
+    private static MailService BuildMailService(InMemoryDatabaseService db)
+    {
+        var signatureService = new Mock<ISignatureService>();
+        var authProvider = new Mock<IAuthenticationProvider>();
+        var mimeFileService = new Mock<IMimeFileService>();
+        mimeFileService
+            .Setup(x => x.SaveMimeMessageAsync(It.IsAny<Guid>(), It.IsAny<MimeMessage>(), It.IsAny<Guid>()))
+            .ReturnsAsync(true);
+        mimeFileService
+            .Setup(x => x.CreateHTMLPreviewVisitor(It.IsAny<MimeMessage>(), It.IsAny<string>()))
+            .Returns<MimeMessage, string>((_, _) => new HtmlPreviewVisitor(string.Empty));
+
+        var preferencesService = new Mock<IPreferencesService>();
+        preferencesService.SetupProperty(x => x.ComposerFont, "Calibri");
+        preferencesService.SetupProperty(x => x.ComposerFontSize, 12);
+
+        var contactPictureFileService = new Mock<IContactPictureFileService>();
+
+        var accountService = new AccountService(
+            db,
+            signatureService.Object,
+            authProvider.Object,
+            mimeFileService.Object,
+            preferencesService.Object,
+            contactPictureFileService.Object);
+
+        var folderService = new FolderService(db, accountService);
+        var contactService = new ContactService(db);
+
+        return new MailService(
+            db,
+            folderService,
+            contactService,
+            accountService,
+            signatureService.Object,
+            mimeFileService.Object,
+            preferencesService.Object);
+    }
+}
diff --git a/Wino.Core/Extensions/OutlookIntegratorExtensions.cs b/Wino.Core/Extensions/OutlookIntegratorExtensions.cs
index 4bfdf542..8ec1407f 100644
--- a/Wino.Core/Extensions/OutlookIntegratorExtensions.cs
+++ b/Wino.Core/Extensions/OutlookIntegratorExtensions.cs
@@ -48,7 +48,7 @@ public static class OutlookIntegratorExtensions
 
         var mailCopy = new MailCopy()
         {
-            MessageId = outlookMessage.InternetMessageId,
+            MessageId = MailHeaderExtensions.NormalizeMessageId(outlookMessage.InternetMessageId),
             IsFlagged = GetIsFlagged(outlookMessage),
             IsFocused = GetIsFocused(outlookMessage),
             Importance = !outlookMessage.Importance.HasValue ? MailImportance.Normal : (MailImportance)outlookMessage.Importance.Value,
@@ -155,7 +155,7 @@ public static class OutlookIntegratorExtensions
             CcRecipients = ccAddresses,
             BccRecipients = bccAddresses,
             From = fromAddress,
-            InternetMessageId = mime.MessageId,
+            InternetMessageId = MailHeaderExtensions.ToHeaderMessageId(mime.MessageId),
             ReplyTo = replyToAddresses,
         };
 
diff --git a/Wino.Core/Synchronizers/GmailSynchronizer.cs b/Wino.Core/Synchronizers/GmailSynchronizer.cs
index df5b27c8..69415630 100644
--- a/Wino.Core/Synchronizers/GmailSynchronizer.cs
+++ b/Wino.Core/Synchronizers/GmailSynchronizer.cs
@@ -2030,13 +2030,13 @@ public class GmailSynchronizer : WinoSynchronizer<IClientServiceRequest, Message
         }
 
         if (string.IsNullOrEmpty(copy.MessageId))
-            copy.MessageId = mime.MessageId;
+            copy.MessageId = MailHeaderExtensions.NormalizeMessageId(mime.Headers[HeaderId.MessageId]);
 
         if (string.IsNullOrEmpty(copy.InReplyTo))
-            copy.InReplyTo = mime.InReplyTo;
+            copy.InReplyTo = MailHeaderExtensions.NormalizeMessageId(mime.InReplyTo);
 
         if (string.IsNullOrEmpty(copy.References) && mime.References?.Count > 0)
-            copy.References = string.Join(";", mime.References);
+            copy.References = MailHeaderExtensions.JoinStoredReferences(mime.References);
 
         if (!copy.HasAttachments && mime.Attachments.Any())
             copy.HasAttachments = true;
diff --git a/Wino.Services/Extensions/MailkitClientExtensions.cs b/Wino.Services/Extensions/MailkitClientExtensions.cs
index 2219f3ee..d42dedd1 100644
--- a/Wino.Services/Extensions/MailkitClientExtensions.cs
+++ b/Wino.Services/Extensions/MailkitClientExtensions.cs
@@ -5,6 +5,7 @@ using MimeKit;
 using Wino.Core.Domain;
 using Wino.Core.Domain.Entities.Mail;
 using Wino.Core.Domain.Enums;
+using Wino.Core.Domain.Extensions;
 
 namespace Wino.Services.Extensions;
 
@@ -64,22 +65,16 @@ public static class MailkitClientExtensions
     }
 
     public static string GetMessageId(this MimeMessage mimeMessage)
-        => mimeMessage.MessageId;
+        => MailHeaderExtensions.NormalizeMessageId(mimeMessage.Headers[HeaderId.MessageId]);
 
     public static string GetReferences(this MessageIdList messageIdList)
-        => string.Join(";", messageIdList);
+        => MailHeaderExtensions.JoinStoredReferences(messageIdList);
 
     public static string GetInReplyTo(this MimeMessage mimeMessage)
     {
         if (mimeMessage.Headers.Contains(HeaderId.InReplyTo))
         {
-            // Normalize if <> brackets are there.
-            var inReplyTo = mimeMessage.Headers[HeaderId.InReplyTo];
-
-            if (inReplyTo.StartsWith("<") && inReplyTo.EndsWith(">"))
-                return inReplyTo.Substring(1, inReplyTo.Length - 2);
-
-            return inReplyTo;
+            return MailHeaderExtensions.NormalizeMessageId(mimeMessage.Headers[HeaderId.InReplyTo]);
         }
 
         return string.Empty;
@@ -109,11 +104,11 @@ public static class MailkitClientExtensions
                            ?? envelope?.Date?.UtcDateTime
                            ?? DateTime.UtcNow;
 
-        var messageId = mime?.GetMessageId() ?? envelope?.MessageId ?? string.Empty;
+        var messageId = MailHeaderExtensions.NormalizeMessageId(mime?.GetMessageId() ?? envelope?.MessageId);
         var fromName = mime != null ? GetActualSenderName(mime) : GetEnvelopeSenderName(envelope);
         var fromAddress = mime != null ? GetActualSenderAddress(mime) : GetEnvelopeSenderAddress(envelope);
         var references = mime?.References?.GetReferences() ?? messageSummary.References?.GetReferences();
-        var inReplyTo = mime != null ? mime.GetInReplyTo() : envelope?.InReplyTo ?? string.Empty;
+        var inReplyTo = MailHeaderExtensions.NormalizeMessageId(mime != null ? mime.GetInReplyTo() : envelope?.InReplyTo);
         var threadId = ResolveThreadId(messageSummary, messageId, references, inReplyTo);
         var hasAttachments = mime != null ? mime.Attachments.Any() : false;
         var itemType = mime != null ? GetMailItemTypeFromMime(mime) : MailItemType.Mail;
diff --git a/Wino.Services/MailService.cs b/Wino.Services/MailService.cs
index d35d8fe1..304cc500 100644
--- a/Wino.Services/MailService.cs
+++ b/Wino.Services/MailService.cs
@@ -14,6 +14,7 @@ using Wino.Core.Domain.Enums;
 using Wino.Core.Domain.Exceptions;
 using Wino.Core.Domain.Extensions;
 using Wino.Core.Domain.Interfaces;
+using Wino.Core.Domain.Misc;
 using Wino.Core.Domain.Models.MailItem;
 using Wino.Messaging.UI;
 using Wino.Services.Extensions;
@@ -82,41 +83,20 @@ public class MailService : BaseDatabaseService, IMailService
             DraftId = $"{Constants.LocalDraftStartPrefix}{Guid.NewGuid()}",
             AssignedFolder = draftFolder,
             AssignedAccount = composerAccount,
-            FileId = Guid.NewGuid()
+            FileId = Guid.NewGuid(),
+            MessageId = GetNormalizedMimeMessageId(createdDraftMimeMessage),
+            InReplyTo = GetNormalizedMimeInReplyTo(createdDraftMimeMessage),
+            References = GetNormalizedMimeReferences(createdDraftMimeMessage)
         };
 
-        // If replying, add In-Reply-To, ThreadId and References per RFC 5322.
-        // References must include all previous References + the Message-ID of the message being replied to.
         if (draftCreationOptions.ReferencedMessage != null)
         {
-            var refMime = draftCreationOptions.ReferencedMessage.MimeMessage;
-            var referenceMailCopy = draftCreationOptions.ReferencedMessage.MailCopy;
-
-            string referenceMessageId = refMime?.MessageId;
-            string referenceInReplyTo = refMime?.InReplyTo;
-            IEnumerable<string> referenceChain = refMime?.References ?? [];
-
-            // Fallback to MailCopy metadata if MIME lacks threading headers.
-            if (string.IsNullOrWhiteSpace(referenceMessageId) && referenceMailCopy != null)
-            {
-                referenceMessageId = referenceMailCopy.MessageId;
-                referenceInReplyTo = referenceMailCopy.InReplyTo;
-                referenceChain = SplitStoredReferences(referenceMailCopy.References);
-            }
-
-            if (!string.IsNullOrWhiteSpace(referenceMessageId))
-                copy.InReplyTo = MailHeaderExtensions.StripAngleBrackets(referenceMessageId);
-
-            var refs = BuildReferencesChain(referenceChain, referenceInReplyTo, referenceMessageId);
-            if (refs.Count > 0)
-                copy.References = string.Join(";", refs);
-
             if (!string.IsNullOrEmpty(draftCreationOptions.ReferencedMessage.MailCopy?.ThreadId))
                 copy.ThreadId = draftCreationOptions.ReferencedMessage.MailCopy.ThreadId;
 
             // Fallback local threading when provider/native thread id is unavailable.
             if (string.IsNullOrWhiteSpace(copy.ThreadId))
-                copy.ThreadId = refs.FirstOrDefault() ?? copy.InReplyTo;
+                copy.ThreadId = MailHeaderExtensions.SplitMessageIds(copy.References).FirstOrDefault() ?? copy.InReplyTo;
         }
 
         await Connection.InsertAsync(copy, typeof(MailCopy));
@@ -997,6 +977,7 @@ public class MailService : BaseDatabaseService, IMailService
         {
             Headers = { { Constants.WinoLocalDraftHeader, Guid.NewGuid().ToString() } },
         };
+        EnsureOutgoingMessageId(message);
 
         var primaryAlias = await _accountService.GetPrimaryAccountAliasAsync(account.Id) ?? throw new MissingAliasException();
 
@@ -1086,6 +1067,7 @@ public class MailService : BaseDatabaseService, IMailService
     {
         var reason = draftCreationOptions.Reason;
         var referenceMessage = draftCreationOptions.ReferencedMessage.MimeMessage;
+        var referenceMailCopy = draftCreationOptions.ReferencedMessage.MailCopy;
         ownAddresses ??= new HashSet<string>(StringComparer.OrdinalIgnoreCase);
 
         var gap = CreateHtmlGap();
@@ -1167,39 +1149,23 @@ public class MailService : BaseDatabaseService, IMailService
                 }
             }
 
-            // Manage "ThreadId-ConversationId"
-            // CRITICAL: In-Reply-To and References headers are essential for threading
-            // They must reference the original message's Message-ID from the MIME headers
-            if (!string.IsNullOrEmpty(referenceMessage.MessageId))
-            {
-                message.InReplyTo = MailHeaderExtensions.StripAngleBrackets(referenceMessage.MessageId);
+            var referenceMessageId = MailHeaderExtensions.NormalizeMessageId(referenceMessage.Headers[HeaderId.MessageId]);
+            if (string.IsNullOrEmpty(referenceMessageId))
+                referenceMessageId = MailHeaderExtensions.NormalizeMessageId(referenceMailCopy?.MessageId);
 
-                var refs = BuildReferencesChain(
-                    referenceMessage.References,
-                    referenceMessage.InReplyTo,
-                    referenceMessage.MessageId);
+            if (!string.IsNullOrEmpty(referenceMessageId))
+            {
+                message.InReplyTo = referenceMessageId;
+
+                var existingReferences = referenceMessage.References?.Select(MailHeaderExtensions.NormalizeMessageId).ToList() ?? [];
+                if (existingReferences.Count == 0)
+                    existingReferences = MailHeaderExtensions.SplitMessageIds(referenceMailCopy?.References).ToList();
+
+                var refs = MailHeaderExtensions.BuildReferencesChain(existingReferences, referenceMessageId);
 
                 foreach (var referenceId in refs)
                     message.References.Add(referenceId);
             }
-            else
-            {
-                // WARNING: Reference message has no Message-ID!
-                // This will break threading. Try to use the MessageId from MailCopy if available.
-                var referenceMailCopy = draftCreationOptions.ReferencedMessage.MailCopy;
-                if (referenceMailCopy != null && !string.IsNullOrEmpty(referenceMailCopy.MessageId))
-                {
-                    message.InReplyTo = MailHeaderExtensions.StripAngleBrackets(referenceMailCopy.MessageId);
-
-                    var refs = BuildReferencesChain(
-                        SplitStoredReferences(referenceMailCopy.References),
-                        referenceMailCopy.InReplyTo,
-                        referenceMailCopy.MessageId);
-
-                    foreach (var referenceId in refs)
-                        message.References.Add(referenceId);
-                }
-            }
 
             if (!string.IsNullOrEmpty(referenceMessage.Subject))
                 message.Headers.Add("Thread-Topic", referenceMessage.Subject);
@@ -1209,8 +1175,8 @@ public class MailService : BaseDatabaseService, IMailService
         var referenceSubject = referenceMessage?.Subject ?? string.Empty;
         if (reason == DraftCreationReason.Forward && !referenceSubject.StartsWith("FW: ", StringComparison.OrdinalIgnoreCase))
             message.Subject = $"FW: {referenceSubject}";
-        else if ((reason == DraftCreationReason.Reply || reason == DraftCreationReason.ReplyAll) && !referenceSubject.StartsWith("RE: ", StringComparison.OrdinalIgnoreCase))
-            message.Subject = $"RE: {referenceSubject}";
+        else if ((reason == DraftCreationReason.Reply || reason == DraftCreationReason.ReplyAll) && !referenceSubject.StartsWith("Re:", StringComparison.OrdinalIgnoreCase))
+            message.Subject = $"Re: {referenceSubject}";
         else
             message.Subject = referenceSubject;
 
@@ -1394,45 +1360,49 @@ public class MailService : BaseDatabaseService, IMailService
         return ownAddresses;
     }
 
-    private static IEnumerable<string> SplitStoredReferences(string references)
+    private static void EnsureOutgoingMessageId(MimeMessage message)
     {
-        if (string.IsNullOrWhiteSpace(references))
-            return [];
+        if (message == null)
+            return;
 
-        return references
-            .Split(new[] { ';', ',', ' ', '\t', '\r', '\n' }, StringSplitOptions.RemoveEmptyEntries)
-            .Select(r => r.Trim());
+        var messageId = MailHeaderExtensions.NormalizeMessageId(MessageIdGenerator.Generate());
+
+        if (string.IsNullOrEmpty(messageId))
+            return;
+
+        var headerValue = MailHeaderExtensions.ToHeaderMessageId(messageId);
+
+        if (message.Headers.Contains(HeaderId.MessageId))
+            message.Headers.Remove(HeaderId.MessageId);
+
+        message.Headers.Add(HeaderId.MessageId, headerValue);
+        message.MessageId = messageId;
     }
 
-    private static List<string> BuildReferencesChain(IEnumerable<string> existingReferences, string parentInReplyTo, string parentMessageId)
+    private static string GetNormalizedMimeMessageId(MimeMessage message)
+        => MailHeaderExtensions.NormalizeMessageId(message?.Headers[HeaderId.MessageId]);
+
+    private static string GetNormalizedMimeInReplyTo(MimeMessage message)
     {
-        var results = new List<string>();
-        var seen = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+        if (message == null)
+            return string.Empty;
 
-        void AddReference(string value)
-        {
-            var normalized = MailHeaderExtensions.StripAngleBrackets(value)?.Trim();
-            if (string.IsNullOrWhiteSpace(normalized))
-                return;
-            if (!seen.Add(normalized))
-                return;
+        var inReplyTo = string.IsNullOrWhiteSpace(message.InReplyTo)
+            ? message.Headers[HeaderId.InReplyTo]
+            : message.InReplyTo;
 
-            results.Add(normalized);
-        }
+        return MailHeaderExtensions.NormalizeMessageId(inReplyTo);
+    }
 
-        if (existingReferences != null)
-        {
-            foreach (var reference in existingReferences)
-                AddReference(reference);
-        }
+    private static string GetNormalizedMimeReferences(MimeMessage message)
+    {
+        if (message == null)
+            return string.Empty;
 
-        // RFC 5322 fallback: if References is absent, include parent In-Reply-To first when available.
-        if (results.Count == 0)
-            AddReference(parentInReplyTo);
+        if (message.References?.Count > 0)
+            return MailHeaderExtensions.JoinStoredReferences(message.References);
 
-        AddReference(parentMessageId);
-
-        return results;
+        return MailHeaderExtensions.NormalizeReferences(message.Headers[HeaderId.References]);
     }
 
     public async Task<IEnumerable<string>> GetRecentMailIdsForFolderAsync(Guid folderId, int count)