Actually I shouldn't call them bots, maybe virus infected zombie computers from all over the world would be more appropriate. Mail is the least of my troubles however ftp hacking has got to the point of me disabling the service :-)
Join the club. I have been getting hammered by bots for the last couple of month with no end in sight. CSF is having a hard time keeping up and I have got to the point of turning services off trying to discourage these lame brains.