This sounds quite bizarre.
In general, if your outgoing mail server has accepted a message from Aqua (and you'd see an error if it hasn't) -- it means it has to deliver the accepted message or bounce it back with an error.
A binding contact, if you will.
Still, I fully believe what you're describing.
Please:
1 - Try updating the main app to the latest development build (this forum has a section with dev builds).
2 - If the issue persists, please contact me for a refund via email (kmansoft / gmail).