Yeah, no edit of sent Messages is allowed. It's a side effect of only allowing one message per turn. Apparantly, it's this way to avoid some sort of exploitation of the AI -- multiple Messages getting sent, and more trades happening, or some such foo.
I really got to admit that I don't really understand the issue. But I still have an opinion on it
. IMHO, MM shouldn't spend so much effort on protecting the AI from exploits. Just playing carefully, with good planning essentialy murders the AI. So exploits might as well be left to our own conscience.
If an 5 star can see a way edited or multiple Messages can ruin a person's day on PBW or email. I'd like to hear it.