Cause 1:
Source object has multiple x500 addresses, and MAGE is using the first encountered address to lookup the target matched Legacy Exchange DN, which would be invalid, as it should be using the source Primary X500 (which MMAD stamps, that should match the target LEDN) to lookup against the target.
This is a known bug at this time (January 11, 2017) with Migration Manager for Exchange & AD, 8.11, 8.12, 8.13. The Fix # for this issue is PT137157685 - There is no Firm ETA for this issue.
Cause 2:
Two conflicting user accounts are created on the target environment. A previous site reported that another user had a UPN that was conflicting with the SMTP address of another user account and caused this generated error.
Resolution 1:
The only "Manual" workaround at this time is:
-Remove all incorrect x500's from the source object, leaving only the primary X500
Resolution 2:
Searching the target Exchange environment may reveal this situation. Resolve it appropriate to the environment, should this be the case.
