Recently while performing a migration from Exchange 2003 to Office 365 (wave 15), I ran into a few mailboxes that were failing the migration with the following error:
The error sounds pretty straightforward, and the general recommendation on the community forums is to delete these accounts and start over. Makes sense… these are user accounts that were created before I started the migration, so maybe they were corrupted? …except for the fact that there were some other users that had previously been created whose mailboxes were migrating just fine. So I checked the accounts in Office 365 and sure enough, their user accounts did not have Exchange Online enabled!
I enabled their Exchange Online licenses, and they were caught in the second round of the migration… might not be the solution for everyone, but definitely a sweet little fix!