Our company is b.com. Our parent company is a.com. We have an Exchange 2013 server. It is configured to receive mail for both domains, and all of our users have email addresses in both domains. When our users receive an email at joe@a.com, the parent company forwards it to joe@b.com. (They use normal DNS lookup; we are not in the same Exchange org/forest/federation as them.) That all works fine.
When our users have their b.com address as their reply address, everything is fine. When their reply address is a.com, everything functions, but Outlook prompts them for a password for user@a.com every few minutes. Entering any combination of credentials or formats has no effect. We used to run Exchange 2007 with no problems, and this began when we installed Exchange 2013 into the environment.
I am guessing it has to do with autodiscover for a.com, which does not and cannot point to our server.
Any thoughts on how to fix this so my users don't have to click X on the password box every 5 minutes all day long?