McAfee VirusScan Access Protection Rules Anti-Virus Standard Protection must be set to prevent mass mailing worms from sending mail.

Many viruses and worms find email addresses on the infected system and send themselves to these addresses. They do this by connecting directly to the email servers whose names they have harvested from the local system. This rule prevents any process from talking to a foreign email server using SMTP. By blocking this communication, a machine may become infected with a new mass-mailing virus, but that virus will be unable to spread further by email. It prevents outbound access to SMTP ports 25 and 587 on all programs except known email clients listed as an exclusion.