During the migration of just one user's mailbox we get:
Exception of type 'Quest.Rest.RestException' was thrown.; WebStatus = Success; StatusCode = ServiceUnavailable; StatusDescription = 'Service Temporarily Unavailable'; Content = '{"message": "Service is temporary unavailable"}'
There's also other log entries indicating MS throttling, such as:
"Waiting for Microsoft Azure to send the response back, The server cannot service this request right now. Try again later. waiting 0.01 minutes before resuming.
This particular mailbox, in both migration attempts, the Microsoft throttling occurred and then the next event to happen was the error and "Failed to migrate mail". And then the task stops.
We had already already relaxed EWS Throttling before the migration. [Though even with the EWS throttling relaxed, MS throttling can still occur, just not as often / strictly as without it having been relaxed.]
There are other migration workloads (not mail migrations) occurring simultaneously.