Could you, please, clarify first a few things:
What messages are you talking about: when talking about "save/reopen": incoming, or those that you are composing?
I suspect it's the latter. If so, could you tell us what you actually do? Are you trying to edit some html-formatted messages/text created in a different app/program?
If so, Aquamail cannot edit formatting created elsewhere.
Now, about forwarded messages: when you are forwarding a message, Aquamail attaches it as is to preserve its formatting . But if you are starting editing that message, IIRC, it strips off all formatting, as it cannot deal with richer HTML formatting.
I am guessing here about your situation, so please describe it in more detail, if I guessed incorrectly.