I'll bet you dollars to donuts that this has nothing to do with Aqua and is caused by something else -- a connectivity issue, an "anti-virus" or "privacy protection" app, or even something I can't even imagine.
Feel free to forward a sample message to support / at / aqua-mail / dot com.
It would be ideal if you could do this as an ".eml attachment" (where the original message is sent as is), this can be done in e.g. Outlook for Windows if you use that.
Second best is to get the message's "original source code" (the headers, the text, everything, as is) and also send as an attachment. Can be done in Gmail web mail, click the icon next to the "reply" icon and select "show original".