its called address harvesting...its one reason why if you are forwarding an email, you should clean off all the previous addresses before sending it on, and if you send emails to groups of people, you should create a group in your email program and not put everyone's address on it....at very least add all the addresses in the bcc line and send it to yourself....for the naughty person harvesting is much easier than hacking account by account....harvested addresses are also sold on.