You are a spam filter, you defend mail inbox. Only good emails are allowed - they are yellow. Spam, viruses and other stuff is not allowed in inbox - kick them out to the spam folder. If you perform too bad you will get uninstalled.
After each wave of mail you can upgrade your gun, each upgrade will cost you some points. There are 20 waves in total. If after beating them you will have some points you can submit them as highscore.