If your legitimate email dealt a lot with replica watches and erectile dysfunction medication, your spam would happen to look a lot more like what you need to see, and I'm pretty sure you'd be unhappy with the current antispam technology...

The issue here is that the offensive text is so close to the desired text, to the point that quoting threatening material makes it non threatening. The tweet 'He said "You will be raped tonight"' may be as non threatening as it gets, while "You will be raped tonight" should land its author before a judge.

I'm not saying it's impossible, but it's IMHO very hard.