Message-Id header changing when sending via ActiveSync

Question

I noticed that when sending mail from Outlook via ActiveSync or SMTP, in some cases (not sure which), the Outlook server changes the Message-Id header on the way out.

This doesn't happen when sending via EWS using the ExtendFieldURI parameter.

Is it possible to guaranty that the Message-Id won't change when sending via ActiveSync or SMTP?

Notes:

1. The Message-Id is generated by the sending mail client
2. This header changes only when sending with ActiveSync or SMTP
3. Looking in the sent items folder, the Message-id is not changed. It is changed only at the receiving side.

Answer 1

I work on Outlook.com. We have a feature that relies on rewriting the Message IDs on the server-side after the submitted message is stored in the mailbox. This is not the case for EWS at this time. There is no way to change this behavior.

Regards, Sean