I’m having an oddly similar but not identical problem. I set up a mailing list on my server that only accepts messages from addresses on the list. It sends me an email telling me to review and accept reject or defer if they’re not on the list.
That way, all my users can post directly to the site; cool.
Problem is, it doesn’t work. When I use the wp-mail.php page, it detects the messages and then deletes them. Somewhere in the middle, it forgets to post them to the site . . .
Anyone have any ideas?