A few months ago I received a strang email from an angry people who said "Stop spamming my guestbook and @#$%%^". At that time I just thought he must contact a wrong person or he himself is a spammer. But today when searching "infrequent keyword site:mysite.com", I found that fake email addresses of my website were used by guestbook/blog comment spammer heavily. The spammer left his website URLs on those guestbook/blog and used fake_and_random_word@mysite.com as his email address. Not only mine, but also @apple.com, @other-company.com and maybe@yours.com. Any ideas? Maybe I should let my ISP know that.