I've created a multi-tenant Exchange 2013 environment and have created a couple of mailboxes for testing.
A user from domain abc.com has an address email@example.com and an additional smtp address firstname.lastname@example.org setup by the applicable EAP.
Another user from domain 123.org has an address email@example.com and an additional smtp address firstname.lastname@example.org setup by the applicable EAP.
Now, these 2 users can email and reply between each other using either user@ or user1@ addresses. The issue comes when I use an external domain from the system altogether and attempt to email their user1@ addresses, I get the following NDR:
Delivery has failed to these recipients or groups:
The email address you entered couldn't be found. Please check the recipient's email address and try to resend the message. If the problem continues, please contact your helpdesk.
Diagnostic information for administrators:
Generating server: HEXS01.testdomain.local
Remote Server returned '< #5.1.1 smtp;550 5.1.1 RESOLVER.ADR.RecipNotFound; not found>'
But these work internal of the HEXS01 server...just not when an external user tries to email the user1@ addresses. When an external user tries to email the user@ addresses they're delivered with no issues.