I'll be happy to take a look if you captured the issue in the app's debug log, see link in my signature below. You will need to enable "raw session data" in debug settings too.
Does this happen with some messages only?
Maybe those that you start in web mail or Outlook, and continue in Aqua?
Can you start a brand new message in Aqua, keep it simple (a line or two of text) and try sending? Will that work?
