Coin grading

He is talking about what are called bots. They are programs that go out and search every web site on the internet looking for email addresses posted just like Mark posted his. Then these bots harvest that email address. The bot owner then takes all those email addresses and sells them to companies large and small all over the world. These compnaies then send out advertisements to all those people.

Or they send messages with phising links in them, that if clicked on, allow the person sending the link to install harmful program on your computer that could be a virus, a worm or a stealth program that records every keystroke you ever make on your computer. This allows them access to your bank accounts, credit card accounts, passwords to every web site you frequent - precisely all the stuff that you don't want anyone else to know.

All that just from posting your email address in a harmless place like this forum.