We have a multi-tenant Exchange 2013 environment. We have a small DAG setup, with two servers ex1 and ex2.
One of our users, on Outlook 2010, keeps getting Undeliverable replies when emailing certain users.
What's interesting is the Undeliverable diagnostic information lists the users that couldn't receive email, as their addresses are incorrect. But at the very bottom of that message, it shows the original header information, with those users' correct address information. So, somehow their addresses are being changed and thus bouncing back.
For example, one of the users is Laura Smith. Her email is email@example.com, but sometimes, when the subject user sends to her, she gets a bounce-back, saying that the user with address LSmith@ex1.hostsubdomain.hostdomain.com does not exist.
Notice that in the above example, Laura's email is switch from LauraS@tenant.com (first name, last initial @ her domain) to LSmith@ex1.hostsubdomain.hostdomain.com (first initial, last name @ ex1's FQDN).
That user's Primary Exchange email is indeed LauraS@tenant.com. She has another email address listed in exchange, which is company_LauraS@hostsubdomain.hostdomain.com, but I cannot understand where or why her address is bouncing back as LSmith@ex1.hostsubdomain.hostdomain.com
Only the subject user is having this issue, no others that I am aware of, so it seems like it may be isolated to either her account or her Outlook profile. We've already done the basic Outlook troubleshooting (new profile, deleting autocomplete cache, among some other little things).
Any idea how I can figure out where and why the address is being changed and incorrectly received by the exchange server?