Exchange will return a message that comes back to the mailbox via a forward. Also, I think, Exchange is doing this because the list is on server and it’s part of Exchange’s deduping. If I send to the list and foo and foo is in the list, foo gets one email, not two.

For example, I have foo@gmail.com, foo@outlook.com, foo@prepend.com.

I set up foo@prepend.com to forward to foo@gmail.com.

If I send a message from bar@gmail.com to foo@prepend.com, it comes through as expected.

If I send the same message from foo@gmail.com, I never get it. Prepend.com’s mail server has a record of sending it to gmail.com, but my gmail account has no evidence of it anywhere.

If I change foo@prepend.com to forward to foo@outlook.com, I don’t have this problem.