This sounds like the same problem I was having (
http://bbs.spamgourmet.com/viewtopic.php?t=218), except mine was when I had set up a trusted sender, not an exclusive sender.
Somone had suggested I just set up the domain as trusted instead of the full email address, which I did; now I am receiving email, through a different spamgourmet address, from a totally different person, which is marked as "from a trusted sender" because the sender is from the same domain - but it's not an email address I actually wanted to be trusted.