Why does Mail generate random new messages in Lion?
Are you running Dragon Express? There's a short discussion of this on the Apple.com forums. I've seen this problem, too, and by Th.e process of elimination have narrowed it down to Express. It seems to keep the mic open even when you aren't using it. You might also see random web searches with the same sort of garbled text.

My guess is that at some point the input buffer fills up and it launches either Mail or Safari with what it's heard.