We are looking at starting a migration of on-premise mailboxes to o365.
Initial test went well apart from mailbox permissions didn't always come over.
What we have found is that if the Exchange 2003 or Exchange 2010 mailbox permission was assigned to a security group that was not mail enabled then it lost the permissions.
Does anyone have a tool or script to go through the mailboxes and calendar and report them ones that aren't mail enabled. over 20000 groups and don't want to have to confirm every non-mailenabled group