We have a shared mailbox on Exchange 2010. This shared mailbox has a forwarding rule that will send any email received to a list of individuals. There is one person who will send an email to the shared mailbox and then will not receive the forward from the shared mailbox (this person is on the recipient list for the mail rule).
We did a mail trace and found that on the emails he sends, he is not on the recipient list.
All other emails received by the shared mailbox are treated as you would expect.