Скачать или смотреть How to Fix Email Threading with MailKit in C# by Properly Setting References and In-Reply-To Heade

Learn how to ensure reply emails sent via MailKit in .NET thread correctly in Gmail and other mail clients by correctly setting Subject, In-Reply-To, and References headers.
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This video is based on the question https://stackoverflow.com/q/79372805/ asked by the user 'Telectrix' ( https://stackoverflow.com/u/11448163/ ) and on the answer https://stackoverflow.com/a/79375113/ provided by the user 'jstedfast' ( https://stackoverflow.com/u/87117/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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The Problem: Email Replies Not Threading Correctly

When replying to the first email in a thread using MailKit in .NET, the recipient sees the reply as a new, separate thread — unlike the Gmail web interface, which threads replies correctly. Interestingly, replying to the latest email in the thread works fine.

This happens because proper email threading depends on specific MIME headers being set correctly, beyond just In-Reply-To.



Understanding Email Threading Headers

Email clients group messages into threads using the following headers:

Subject: Should closely match the original message subject, often prefixed with "Re: "

In-Reply-To: References the Message-Id of the message being replied to

References: A chain of all prior message IDs in the conversation

Incorrect or incomplete setup of these headers causes email clients (like Gmail) to display replies as new threads.



How to Correctly Send a Reply Using MailKit

Prefix the Subject with "Re: "

Always use the original message's Subject, prepended by "Re: "

[[See Video to Reveal this Text or Code Snippet]]

Set the In-Reply-To header

Assign it exactly to the Message-Id of the message you are replying to

[[See Video to Reveal this Text or Code Snippet]]

Include all original References

Copy every reference from the original message to maintain the thread history

[[See Video to Reveal this Text or Code Snippet]]

Append the original message's Message-Id to References

Add the current replied message's ID to extend the reference chain

[[See Video to Reveal this Text or Code Snippet]]

Send the email as usual using SmtpClient



Why This Works

By preserving the full chain of references and setting the In-Reply-To header properly, you ensure your reply is recognized as part of an existing conversation. This matches the behavior of Gmail's web client and other major mail clients.

Summary Checklist for Threaded Replies

[x] Subject prefixed with "Re: "

[x] In-Reply-To set to original Message-Id

[x] Copy all References from original message

[x] Append original Message-Id to References

Adhering to these steps will make your MailKit replies thread properly, even when replying to the first message in a thread.