To add my two cents:
If there were no errors when sending, and the messages are seen as sent in web mail -- it means the outgoing mail server accepted your message from Aqua.
After that, the message should either get delivered, or there should be a "bounce" message explaining why not.
Things to try in Aqua:
- Settings -> composing and sending
- Turn off "Send User-Agent"
If it helps, it means someone someone has a "spam" filter with "AquaMail" in it.