Got Exchange 2007 and also an Exchange 2013 server (in the middle of migration). From what I understand, 2007 hands off to 2013 for all sending of email.
Recently, we had an issue where some emails to one domain were getting bounced but some users were getting through. It turned out that users with contacts were working but those without were being bounced. In the end, discovered that the remote domain was set up as an authoritative domain in Exchange 2007 which was causing the problem. Removed this and it all worked.
But, we don't know why it suddenly stopped working. Couple of points:-
1. Pretty sure the remote domain setting in exchange 2007 has been there for a while.
2. This used to work until about two week until I deleted the authorative domain.
My theory is that something has been changed on Exchange 2013 that makes this authoritative domain setting "relevant". I.e. It goes from not caring to restricting to contact only users.
So far no-one has admitted any changes on Exchange 2013 :-( (Its not just me who looks after). We're desperately trying to work out exactly what has happened here to prevent accidentally doing this again.
Any idea what change on 2013 would make this setting relevant?