Hi Mikhail,
In my opinion, when MFA applies the retention policy in the mailbox, it will stamp items in the mailbox and inspect them, if the items past their retention age, MFA will take the specified retention action on them, the retention action will process these
items, like move to archive, delete an so on.
The process of these actions do not use any protocol (like SMTP, MAPI..), Exchange will run it in the background processing, just like copy and paste item between two folders.
Best regards,
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Niko Cheng
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