When Migrating the following message is seen in the logs:
The Exchange mailbox database is temporarily unavailable, probably due to routine maintenance.
This is a transient error and ODME will reschedule the mailbox and they this again after a set amount of time. ODME will do this per migration up to 100 times.
If ODME is not able to complete the migration successfully after 100 reschedules, it is best to copy the affected users into a new plan and migrate these users from this new plan so the users that have migrated successfully do not have to be migrated again.
Please see KB110994 for instructions on copying users into a new plan.