The science behind the service that sifts through your spam. Junk mail is detected by scanning the message for common warning signs. Was the message sent from a blacklisted network? Have the headers been tampered with to hide the sender? Does the subject line use all capital letters? All these factors are added together to give a weighted score, and anything over a certain number is deemed to be spam. If you manually mark an email as spam, it also refines the scoring system for future emails.