The error below is recorded:
An existing connection was forcibly closed by the remote host
The error may be present in both the Console and Mage.log.
Where large individual mail items are suspected or confirmed reducing the MaxItemCountPerBatchUploadToExchange (detailed in https://support.quest.com/technical-documents/migration-manager-for-exchange/8.14/user-guide/22#TOPIC-529190) can positively help clear the error.
The default setting is 32, reducing this initially to 16, re-sync the mailbox/s and monitor the progress of the sync.
Further adjustments downward may be required dependent on the amount of large emails required to process.
