I have received reports recently from people who are getting odd spam messages delivered to them that don't actually try to sell them prescription drugs, visas to the US, methods of enlarging his or her body parts, or cheap loans so they can refinance his or her home. Instead of these commonly known scams, the spam messages in question use a recipient's own email address as the return address, and have a subject line and message body containing random numbers. No exploit inside, no malicious code, no links.
Initially, a lot of theories were put forth; from spam software gone wrong, to spammers trying to poison Bayesian spam filters. It turns out the reason for these odd spam messages is nothing other than a familiar mass mailing worm, Beagle. W32.Beagle.FC is another variant of the Beagle family. Beagle is split into many components: one component may just try to download other components, others gather up email address, and the remaining components actually send Beagle to email addresses it has discovered.
W32.Beagle.FC is using a new technique to try to validate email addresses before continuing. The threat simply sends out these innocuous email messages and checks the SMTP return code. If the mail server states that the email address doesn't exist, it dumps the address. If the email appears to be successfully sent, the email address is saved. This allows Beagle to verify if email addresses are live (the technique isn't fool proof, as email servers may accept some mails even though no one is on the other end) so it can build a pretty reputable list of email addresses for future use.
How are these validated email addresses going to be used in the future? We don't know just yet, but the likelihood is that they will be used for spamming out malicious code or spam itself. So, if you are looking to refinance your debts, enlarge a body part, take some blue pills, or move to the US, don't worry: an offer is on its way.