Most emails to my host bouncing

To go along with my post about pine, most (but seemingly not all) emails to my host are bouncing back to the sender, saying “unknown user: ‘foo’”, where foo is the intended recipient.

To self-followup, it would appear that users who also have an email forward (we all check email normally via pine, but some send email off to gmail for archival) have the forwarded emails reaching their destination.