Surely google can correlate the contents of your mail with blogs on getting fired/splitting up with your partner/falling out with your relatives and advise you whether or not you should send it.
Funny you mention that, I used to have a Mac mail app (forget which) that had a feature like this. It did some kind of keyword counting thing and would display one, two, or three chili peppers next to inbound and outbound emails.
There was some scripting built in, and one of the things you could do was add a one hour sending delay to anything with a certain number of chilis.
The combination of visual display and scripted "let me sleep on it" features worked really well.
The chili-peppers language-analysis is Eudora's "Mood Watch" feature. Never used it, but always thought it was a good idea.
Similarly, email clients should catch references to attachments that aren't there, and ask "do you really want to send this without an attachment?"
Even more generally, they could recognize (and allow to be configured) certain addresses as requiring additional confirmation before sending -- so a mistaken overbroad or misdirected CC becomes less common.
(I'm thinking: first time you send to an address, you get a confirmation popup, and a chance to classify this address: require dialog confirm; require captcha/retype-of-name confirm; require waiting period; require confirm if combined with other address; etc.)
