Return to sender, false address unknown

Spammer technique or marketing ploy?

Are spammers deliberately getting mail servers to bounce undeliverable messages towards their targets as a way of getting their junk read?

Computer Mail Services (CMS), a Michigan-based e-mail security and management software provider, certainly thinks so and reckons the "Reverse Non-Delivery Report" (RNDR) technique is being used by spammers to steal server capacity and avoid detection.

However a leading spam catching expert said the technique is only rarely used by spammers. And when we consider that CMS sells a anti-spam product - called Praetor - which blocks the technique (the firm suggests mail servers are vulnerable too) - our suspicions are further aroused. The closer one looks into CMS' alert, the more it looks like a marketing ploy.

Doubts aside, what does the RNDR technique involve anyway? Over to Computer Mail Service for an explanation:

A Reverse Non-Delivery Report (RNDR) attack occurs when the spammer takes advantage of a server's inherent ability to return email that is misaddressed. Typically, when misaddressed email is sent, the mail is returned to the originating sender along with an error notification and the original message. By forging the originator's address and sending to fictitious addresses within the domain of the victimised enterprise's servers, the spammer can send emails to thousands of intended addressees without his own server ever being detected.

Matt Sergeant, senior anti-spam technologist at managed services firm MessageLabs, said it has only rarely seen the technique used by spammers.

"This doesn't seem to be a technique to get spam out, though it does offer a simple way for spammers to avoid been detected. We see some of spam email like this but it's down at the noise level," he told The Register.

According to Sargeant, one of the disadvantages of the technique is that mail servers commonly do not include the subject line of bounced messages. He believes this reduces the chance of spam messages been opened. CMS says the opposite is true.

Open relay vulnerabilities (such as this) which permit spammers to co-opt corporate servers for their nefarious purposes pose a far greater risk in practice than these "bounced message" techniques, we believe.

Make up your own mind how serious a problem this is by reviewing CMS' alert / sales pitch here.

Spam nuisance growing

However spam arises, evidence is increasing that shows the problem is getting worse.

The latest monthly statistics from anti-spam firm Brightmail show that 46 per cent of the messages it processed in May turned out to be spam. Brightmail's findings are roughly in line with those of MessageLabs which reports that spam email exceeded legitimate email in the messages it processed for the first time last month. ®

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