When they send you an email, they already have your email address. It's just a matter of confirming that you open and read their emails which makes you a target for them. If you read their emails, you are more likely to buy viagra.

Before I spam you, I would have to craft the email to include a tag like
<img src="[email protected]"?>

They would then have your IP, and "proof" that you opened the email. You can also get an IP by watching requests on your webserver, but using php will give you an email, an IP, and a way to put it in email notification like my tutorial, or you could stick it in a DB which is more likely on the spammer end.