So, this is an odd one. I have a user (I call User A) that can send email to anyone except one person (User B). I can login to User A, and send an email addressed to User B, C, D, etc, and everyone will get it except user B. User B can send an email to User A just fine. Neither one have any filters setup. Watching the logs, whenever User A sends an email to B, it shows in the logs that an email was sent from User A, but nothing is recorded at all for user User B.

The mail is added to the sent folder, nothing is bounced back, it's like it just disappears into the ether.

Does anyone have an idea what's going on?