Below you will find three possible workaround which were used in past.
WORKAROUND 1:
It has been confirmed that in most environments if you cleanup this attribute the RUS will populate the value correctly. Quest tool "Active Directory Cleanup Utility", which is mentioned in this KB article:
https://support.quest.com/kb/9980/how-can-i-clean-up-the-service-attributes-used-by-directory-synchronization-agent-dsa-during-th-9980- can be used for clean up purposes.
*****This approach should be tested in a lab before implementing in production*****
WORKAROUND 2:
ADModify and another tools and utilities or scripts can be used to change the format of the msExchHomeServerName attribute back to the proper value.
This approach could be cumbersome if target environment has multiple Exchange servers because the msExchHomeServerName attribute value is unique to each server and contains the server name.
RESOLUTION:
A patch has been released to fix this issue, to obtain the Hotfix please contact Quest Technical Support and reference Hotfix "MigrationManager7.XUpdate20061123".
Please note: This patch only updates the servers you specify. Any new servers that are brought online will have to be updated, and you will have to update the server any time the shared components or a mail agent are re-installed.
STATUS:
Waiting for fix in a future release of Migration Manager for Exchange.