I have SBS 2008 Server, with Exchange 2007. This issue involves three users.
They had accounts and email associated with the primary domain (mappingalaska.com). They also had alias account set up for a different domain, hosted elsewhere (gpsalaska.com). In short, they worked for two companies, but no longer work for the primary domain so their account were deleted. We still have users that work for the primary domain, with alias accounts to the outside domain.
The problem is that I receive NDR's when attempting to send mail to them. "#550 5.1.1 RESOLVER.ADR.RecipNotFound; not found ##". They can receive email if it originated form outside of the primary (mappingalaska.com) domain.
I have recreated the accounts on the primary (mappingalaska.com) domain and added the alias email address to their profiles. But they still don't receive the messages originating from the primary domain, but the NDR's have stopped. I have renamed the *.NK2 files in AppData, but that has not made a difference.