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We encounter very strange behavior with the Exchange 365/Outlook 365 combination. When sending e-mails between 2 Exchange users on the same server, internet message id (we checking PR_INTERNET_MESSAGE_ID using OutlookSpy) is different for the message in the sender Sent folder and recipient's Inbox folder. It works ok when Outlook is in cached mode and when sending messages using OWA. Only Outlook in online-mode is affected.

Any clues what might be wrong?

Exchange version is: 15.20.3391.14

Outlook is the latest version from Microsoft 365 subscription

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Did you use Exchange Online?

I suggest you use MCFMAPI to check the message id. Did you have any 3rd party software? If yes, please disable them and check again. You also could use message tracking to check which process change it.

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  • Yes, I use Exchange Online. I check the message-id with OutlookSpy. No 3rd party software except it. How can we use message tracking with online Exchange? I thought it is available only for on-premise Exchange. Oct 21, 2020 at 13:45
  • Yes, you could refer to blog.edbmails.com/…
    – Jayce
    Oct 22, 2020 at 8:06

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