An email address that you give when you don't want anyone to know your real email address. Most commonly used when you have to register at a site to view porn. Also, having a porn address allows you the freedom and benefit of giving your girlfriend access to your email without worry.
"Dude, I met this SMOKING hot chick on the plane today and she wants to hook up!"
"Whoa, aren't you worried Shirley will find out?"
"Nah, I gave her my porn address."