The error below appears in the Mage.log.
"Microsoft.Exchange.WebServices.Data.ServerBusyException: The server can not process this request now. Try again later error "
The problem mailbox will neither finishing sync or switch.
Project size limitations have been increased in accordance with https://support.quest.com/kb/213729 but the problem persists
When project options SizeLimitInMbPerBatchUploadToO365 and SizeLimitInMbPerBatchUploadToExchange (referred in https://support.quest.com/kb/213729) have been amended and can be ruled out as the potential cause please consider the following steps:
1) Create a new collection to contain the problematic mailbox/s.
2) Set the -ClearAllSyncDataOnResync option on this new collection (see for https://support.quest.com/technical-documents/migration-manager-for-exchange/8.13/user-guide/22#TOPIC-529190 for instructions).
3) Move the problem mailbox/s to the new collection created in step 1.
4) Perform a re-synchronize action on the problem mailbox/s.