Problem

During a migration to Office 365, old appointments can generate unattended meeting alerts. This is due to a behavior of Exchange; an alert is sent to all attendees to warn them that the date of the meeting is no longer valid: “Your meeting was found to be out of date and has been automatically updated”.

Cause

Sometimes, old appointments include the file winmail.dat. This file triggers a process in Exchange and as a result, mails are sent to the attendees.

Example:
Content-Disposition: attachment; filename="winmail.dat" Content-Transfer-Encoding: base64 Content-Type: application/ms-tnef; name="winmail.dat"

Resolution

Create a transport rule in your Exchange admin center to delete this kind of message:

Exchange rule for unattended meeting alert