While migrating one AD domain to another, I stumbled into the legacyExchangeDN problem.
We had a lot of of global mail contacts in the first domain pointing to recipients in the other domain and vice versa. Many users had cached contacts in Outlook that ceased to work after migrating to the other domain generating IMCEAEX non-delivery reports.
So we had to fix this by adding the legacyExchangeDN string of the mail contacts as a X500 proxy address.
Luckily this does not affect mailboxes migrated using Exchange's forest migration feature. The legacyExchangeDN is properly migrated.
Some good information on this topic:
Published on 2022-02-17, 20:45 +0000