We've been doing a migration recently from Gmail - about 120 users.
There are a handful of those mailboxes that migrate everything except for the emails. Calendar, tasks and contacts all come over fine. If I try the migration myself, I can watch the emails coming across in the imapretrieval folder, but as soon as all messages are brought it, the folder is deleted and nothing new shows up in the users mailbox.
There are no errors, etc. to indicate a problem.
I have not yet restarted the mail server - have to schedule a maintenance window. Going to try that this weekend.