Mail flow NDR to external email addresses that have Active Directory Accounts.

Hello,

We are running an Exchange Server 2016 (Version 15.1 ‎(Build 2176.2)‎).

We noticed recently that staff emailing a few external email accounts were receiving NDR's. Upon further examination, we found that these external email addresses did indeed contain an Active Directory account for the user, although there was no local mailbox on Exchange, nor a Contact.  These users access our network only to use certain applications.

We found that even when the email address was being manually typed in Outlook, it was trying to use the Active Directory account as the destination. The Active Directory account contained an entry in the General tab that contained the external email address.


For the most part, the issue was addressed by removing the email entry from the General tab. In some cases, I had to create an associated mailbox Then create a Contact with the external email address. Then forward the internal email to the Contact.

My question is what are the Microsoft best practices in this situation?
1. Should the General Tab NOT contain an email address if it is for an external mailbox, even if there is no internal mailbox?
2. Does Outlook try to route email internally using this entry even if it is not setup as an email domain that it accepts email for?

Thanks in advance for the assistance.

Regards,
Real-Timer