Used to describe e-mail. Came from the rebellious style of speak that uses words like "has" and "are" in places that they normally wouldn't be used. It really doens't have much of a purpose, read the examples - it's the only way to understand. Could be used as a noun or a verb.
Normal: "Hey, you have mail"
better: "Hey, you has mail"
hasmail: "Hey, you hasmail"
hasmail: "Hey, go check your hasmail"
"I just sent you some hasmail"