i use a mod called SimpleMail right now, that remembers the last person a mail was sent to, and leaves it in the To: box for your next mail. that's all it does, all it needs to do!
the mod is extremely outdated (february) and i'm sure an ace addon author could do a much cleaner job of rewriting it, and supporting it for us in the future.
it does work now, but i'm worried about the upcoming patches, and no support, etc.
here's a link to the mod: SimpleMail
thanks a lot!