MicrosoftTimo_Strueber when we implemented the muli-tenant scenario there was no need to perform any action about "cross-tenant" LegacyExchangeDN.
In fact we noticed that once recipients has been moved to different tenants, even when replying to old messages, received once everybody were in same on-prem environment. Outlook used the smtp address in any case.
So no need to deal with x500 and legacy addresses.
We have found only an exception with calendar/Teams meeting. If a calendar was sent before the migration and after the migration a user fwd or reply to it. It was looking for the LegacyExchangeDN of the original sender generating NDR. But in our project we have considered this specific use case an accepted risk.
