diff --git a/Wino.Core.Domain/Extensions/MailHeaderExtensions.cs b/Wino.Core.Domain/Extensions/MailHeaderExtensions.cs
new file mode 100644
index 00000000..aa41729b
--- /dev/null
+++ b/Wino.Core.Domain/Extensions/MailHeaderExtensions.cs
@@ -0,0 +1,42 @@
+using System;
+using System.Linq;
+
+namespace Wino.Core.Domain.Extensions;
+
+public static class MailHeaderExtensions
+{
+    /// <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
+    /// properties store them without brackets. This normalizes raw header
+    /// values to match MimeKit's convention.
+    /// </summary>
+    public static string StripAngleBrackets(string value)
+    {
+        if (string.IsNullOrEmpty(value)) return value;
+
+        value = value.Trim();
+
+        if (value.StartsWith("<") && value.EndsWith(">"))
+            return value.Substring(1, value.Length - 2);
+
+        return value;
+    }
+
+    /// <summary>
+    /// Normalizes a raw RFC References header value into semicolon-separated Message-IDs
+    /// without angle brackets. Raw References headers contain space-separated bracketed IDs
+    /// like "&lt;id1@domain&gt; &lt;id2@domain&gt;". This converts them to "id1@domain;id2@domain".
+    /// </summary>
+    public static string NormalizeReferences(string rawReferences)
+    {
+        if (string.IsNullOrEmpty(rawReferences)) return rawReferences;
+
+        var ids = rawReferences
+            .Split(new[] { ' ', '\t', '\r', '\n', ';', ',' }, StringSplitOptions.RemoveEmptyEntries)
+            .Select(StripAngleBrackets)
+            .Where(id => !string.IsNullOrEmpty(id));
+
+        return string.Join(";", ids);
+    }
+}
diff --git a/Wino.Core/Extensions/OutlookIntegratorExtensions.cs b/Wino.Core/Extensions/OutlookIntegratorExtensions.cs
index 5036f2b8..894b1bce 100644
--- a/Wino.Core/Extensions/OutlookIntegratorExtensions.cs
+++ b/Wino.Core/Extensions/OutlookIntegratorExtensions.cs
@@ -8,6 +8,7 @@ using Wino.Core.Domain.Entities.Calendar;
 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.Misc;
 
 namespace Wino.Core.Extensions;
@@ -64,6 +65,20 @@ public static class OutlookIntegratorExtensions
             ItemType = MailItemType.Mail // ItemType will be set by caller if calendar access is granted
         };
 
+        // Extract In-Reply-To and References from InternetMessageHeaders for threading.
+        if (outlookMessage.InternetMessageHeaders != null)
+        {
+            var inReplyToHeader = outlookMessage.InternetMessageHeaders
+                .FirstOrDefault(h => string.Equals(h.Name, "In-Reply-To", StringComparison.OrdinalIgnoreCase));
+            if (inReplyToHeader != null)
+                mailCopy.InReplyTo = MailHeaderExtensions.StripAngleBrackets(inReplyToHeader.Value);
+
+            var referencesHeader = outlookMessage.InternetMessageHeaders
+                .FirstOrDefault(h => string.Equals(h.Name, "References", StringComparison.OrdinalIgnoreCase));
+            if (referencesHeader != null)
+                mailCopy.References = MailHeaderExtensions.NormalizeReferences(referencesHeader.Value);
+        }
+
         if (mailCopy.IsDraft)
             mailCopy.DraftId = mailCopy.ThreadId;
 
@@ -134,7 +149,7 @@ public static class OutlookIntegratorExtensions
             CcRecipients = ccAddresses,
             BccRecipients = bccAddresses,
             From = fromAddress,
-            InternetMessageId = GetProperId(mime.MessageId),
+            InternetMessageId = mime.MessageId,
             ReplyTo = replyToAddresses,
             Attachments = []
         };
@@ -429,16 +444,6 @@ public static class OutlookIntegratorExtensions
         return headers;
     }
 
-    private static string GetProperId(string id)
-    {
-        // Outlook requires some identifiers to start with "X-" or "x-".
-        if (string.IsNullOrEmpty(id)) return string.Empty;
-
-        if (!id.StartsWith("x-") || !id.StartsWith("X-"))
-            return $"X-{id}";
-
-        return id;
-    }
 
 
     #endregion
diff --git a/Wino.Core/Synchronizers/GmailSynchronizer.cs b/Wino.Core/Synchronizers/GmailSynchronizer.cs
index b390a9a6..74023de5 100644
--- a/Wino.Core/Synchronizers/GmailSynchronizer.cs
+++ b/Wino.Core/Synchronizers/GmailSynchronizer.cs
@@ -25,6 +25,7 @@ using Wino.Core.Domain.Entities.Mail;
 using Wino.Core.Domain.Entities.Shared;
 using Wino.Core.Domain.Enums;
 using Wino.Core.Domain.Exceptions;
+using Wino.Core.Domain.Extensions;
 using Wino.Core.Domain.Interfaces;
 using Wino.Core.Domain.Models.Accounts;
 using Wino.Core.Domain.Models.Folders;
@@ -1691,9 +1692,9 @@ public class GmailSynchronizer : WinoSynchronizer<IClientServiceRequest, Message
             IsRead = !isUnread,
             IsFlagged = isFlagged,
             IsFocused = isFocused,
-            InReplyTo = gmailMessage.Payload?.Headers?.FirstOrDefault(h => h.Name.Equals("In-Reply-To", StringComparison.OrdinalIgnoreCase))?.Value,
-            MessageId = gmailMessage.Payload?.Headers?.FirstOrDefault(h => h.Name.Equals("Message-Id", StringComparison.OrdinalIgnoreCase))?.Value,
-            References = gmailMessage.Payload?.Headers?.FirstOrDefault(h => h.Name.Equals("References", StringComparison.OrdinalIgnoreCase))?.Value,
+            InReplyTo = MailHeaderExtensions.StripAngleBrackets(gmailMessage.Payload?.Headers?.FirstOrDefault(h => h.Name.Equals("In-Reply-To", StringComparison.OrdinalIgnoreCase))?.Value),
+            MessageId = MailHeaderExtensions.StripAngleBrackets(gmailMessage.Payload?.Headers?.FirstOrDefault(h => h.Name.Equals("Message-Id", StringComparison.OrdinalIgnoreCase))?.Value),
+            References = MailHeaderExtensions.NormalizeReferences(gmailMessage.Payload?.Headers?.FirstOrDefault(h => h.Name.Equals("References", StringComparison.OrdinalIgnoreCase))?.Value),
             FileId = Guid.NewGuid(),
             ItemType = itemType
         };
diff --git a/Wino.Core/Synchronizers/ImapSynchronizer.cs b/Wino.Core/Synchronizers/ImapSynchronizer.cs
index 2fc17dbf..1ac92063 100644
--- a/Wino.Core/Synchronizers/ImapSynchronizer.cs
+++ b/Wino.Core/Synchronizers/ImapSynchronizer.cs
@@ -196,6 +196,9 @@ public class ImapSynchronizer : WinoSynchronizer<ImapRequest, ImapMessageCreatio
             if (!smtpClient.IsAuthenticated)
                 await smtpClient.AuthenticateAsync(Account.ServerInformation.OutgoingServerUsername, Account.ServerInformation.OutgoingServerPassword);
 
+            // Remove local draft header before sending to prevent leaking to recipients.
+            singleRequest.Mime.Headers.Remove(Domain.Constants.WinoLocalDraftHeader);
+
             // TODO: Transfer progress implementation as popup in the UI.
             await smtpClient.SendAsync(singleRequest.Mime, default);
             await smtpClient.DisconnectAsync(true);
@@ -218,10 +221,6 @@ public class ImapSynchronizer : WinoSynchronizer<ImapRequest, ImapMessageCreatio
                 var sentFolder = await client.GetFolderAsync(singleRequest.SentFolder.RemoteFolderId);
 
                 await sentFolder.OpenAsync(FolderAccess.ReadWrite);
-
-                // Delete local Wino draft header. Otherwise mapping will be applied on re-sync.
-                singleRequest.Mime.Headers.Remove(Domain.Constants.WinoLocalDraftHeader);
-
                 await sentFolder.AppendAsync(singleRequest.Mime, MessageFlags.Seen);
                 await sentFolder.CloseAsync();
             }
diff --git a/Wino.Services/MailService.cs b/Wino.Services/MailService.cs
index 5b248a8a..e5323775 100644
--- a/Wino.Services/MailService.cs
+++ b/Wino.Services/MailService.cs
@@ -86,14 +86,24 @@ public class MailService : BaseDatabaseService, IMailService
             FileId = Guid.NewGuid()
         };
 
-        // If replying, add In-Reply-To, ThreadId and References.
+        // 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)
         {
-            if (draftCreationOptions.ReferencedMessage.MimeMessage.References != null)
-                copy.References = string.Join(",", draftCreationOptions.ReferencedMessage.MimeMessage.References);
+            var refMime = draftCreationOptions.ReferencedMessage.MimeMessage;
+            var refs = new List<string>();
 
-            if (!string.IsNullOrEmpty(draftCreationOptions.ReferencedMessage.MimeMessage.MessageId))
-                copy.InReplyTo = draftCreationOptions.ReferencedMessage.MimeMessage.MessageId;
+            if (refMime.References != null)
+                refs.AddRange(refMime.References);
+
+            if (!string.IsNullOrEmpty(refMime.MessageId))
+            {
+                copy.InReplyTo = refMime.MessageId;
+                refs.Add(refMime.MessageId);
+            }
+
+            if (refs.Count > 0)
+                copy.References = string.Join(";", refs);
 
             if (!string.IsNullOrEmpty(draftCreationOptions.ReferencedMessage.MailCopy?.ThreadId))
                 copy.ThreadId = draftCreationOptions.ReferencedMessage.MailCopy.ThreadId;
@@ -960,8 +970,8 @@ public class MailService : BaseDatabaseService, IMailService
 
                     if (!string.IsNullOrEmpty(referenceMailCopy.References))
                     {
-                        // Parse the References string and add them
-                        var references = referenceMailCopy.References.Split(new[] { ',' }, StringSplitOptions.RemoveEmptyEntries);
+                        // Parse the References string (supports both ";" and "," separators for backward compatibility)
+                        var references = referenceMailCopy.References.Split(new[] { ';', ',' }, StringSplitOptions.RemoveEmptyEntries);
                         foreach (var reference in references)
                         {
                             message.References.Add(reference.Trim());