Ha! No – sorry. I shouldn’t laugh. It is one of those silly things that you don’t think of…
If the post content is totally empty then new posts are rejected. This makes total sense of course. With PBE and IMAP, an attachment is NOT held and returned as a part of the content of the email. So in your case it just saw an empty email and stopped processing.
I will open a bug ticket and get this fixed up.