Hello,
Sorry to hear this, certainly rare these days, although I don't question your experience.
Could you try it on the first phone again, let it work for a few hours, and then check system settings -> battery usage? That screen lists all battery consumers, apps among them, let's see if AquaMail is listed there with a high consumption level.
If it is, please enable debug logging (see my signature below) and leave it alone for a couple of hours, then sent me the log.
Thanks in advance.