Theft of service

Spammers frequently seek out and make use of vulnerable third-party systems such as open mail relays and open proxy servers. SMTP forwards mail from one server to another—mail servers that ISPs run commonly require some form of authentication to ensure that the user is a customer of that ISP. Open relays, however, do not properly check who is using the mail server and pass all mail to the destination address, making it harder to track down spammers.

Increasingly, spammers use networks of malware-infected PCs (zombies) to send their spam. Zombie networks are also known as Botnets (such zombifying malware is known as a bot, short for robot). In June 2006, an estimated 80% of e-mail spam was sent by zombie PCs, an increase of 30% from the prior year. An estimated 55 billion e-mail spam were sent each day in June 2006, an increase of 25 billion per day from June 2005.