I've just been bitten by this due to a temporary network outage. I spent most of the evening on and off composing a long epic response to a thoughtful mail. While I was doing so my network flapped a few times, but I thought nothing of it. When I came to actually send the mail... no dice. Vim closed normally, the temp file was deleted, and I was then informed that I had no connection to my SMTP server.
I tried to recreate this by sending a mail when my network was artificially down and encountered a similar failure, but at least in that case the email was left in /tmp and could be retrieved. Not so with the one I actually care about.
I've lost an evening's thought, which I will have to try and recreate tomorrow, with no warning.
I admit I haven't looked at the code, but if graceful reconnection is difficult perhaps it would be possible to save mails somewhere local / delete the temp file only after the mail has been confirmed sent / at least display an obvious banner warning that aerc is dead walking even though the user is composing a mail and give them the opportunity to manually save it somewhere.