Tackling “Undelivered Mail return to sender” and Image Spam with Greylisting
Over 90% of email is now junk mail according to a recent BCS article.
Not only are spam volumes increasing, but junk messages are proving harder to filter. Some spam buries its message in images to prevent filtering on content. To reduce the number of these in your inbox, quarantine messages with headers containing “Content-Type: multipart/related” but check your quarantine folder regularly for false positives.
Others send spam for you to a third party and forge the sender’s address so that it appears to have come from you. This technique is known as backscatter. If you keep receiving messages with the header “Undelivered Mail return to sender” about emails you never sent then you may be the victim of this technique. Quaranting messages containing “Action: failed”, “Delivery Status Notification (Failure)” and/or with the subject containing “Undeliverable” will help reduce the volume of these although again this risks filtering off genuine messages alerting you to a failed delivery.
I’ve just started testing greylisting on my email. Email from unknown senders is temporarily bounced back: legitimate mail clients will try again later; spammers either will not try again or hopefully will have been added to a blacklist by the time they do so. Known senders are added to a whitelist and automatically bypass the greylist filter.
Mailsnare offer server-side greylisting. However, I have been disappointed with their service levels recently and am not sure how strongly I would recommend them.
March 14th, 2008 at 4:50 am
Tim, How does this greylisting work when the email that comes back to you is thought to have originated from you and hence can’t be stopped by normal junk mail listings. I get the Undelivered emails and when I try to mark it as junk mail, it states it has come from my server and hence won’t allow me to mark it as junk mail. Thanks, Craig.
April 24th, 2008 at 12:40 am
Hi Craig - sorry for the delay in responding. Your comment slipped through the cracks. You can’t greylist backscatter unfortunately which is why you need to write a manual filter based on the typical message contents of an Undelivered and either send them to your spam folder (not quite the same as marking as spam but it has the same end result) or delete them automatically. Hope this helps.