email returned as undeliverable, but not in system

I sent an email to members of Quest 4643 and one came back. It was addressed to alireza.x.kasra@questdiagnostics.com, but this address is not in our system. Ali's address is alikas12000@yahoo.com and he gets emails from us all the time. Even if he had his email forwarded to the questdiagnostics email, it would already be out of the FTH server's control, right?
Any idea how this happened?

Again, this is for Quest 4643.

For bounces from "end recipients", we do route some of those back to the original sender if the server code can determine who the original sender was. The reason is that the bounce from the end recipient will come back to the last sender, which will always be the server since it is the "middleman" (the email relay / forwarder). If the list server can figure out who the original sender was, it will then relay the bounce back. In this sense, the server is serving as a relay "in the reverse direction".

You also need to keep in mind that each member can set both a primary and secondary email address, and you only see the primary email address in Membership Management. Therefore, when troubleshooting email issues with "unknown" email addresses, you should also always inspect the persons actual member profile to see if there is a secondary email address in use. In this case, alireza.x.kasra@questdiagnostics.com is the person's secondary email address as shown in their profile. It is likely bad or non-functioning.