What is it?
Backscatter is a certain kind of mail you receive that you didn't ask to receive. If you've ever received a “Your mail could not be delivered” bounce notification, a “Your mail contained a virus” notice, or a request to confirm your signup request for a mailing list you've never heard of, you've probably received backscatter. The backscatter problem is inherently linked to the spam problem, as most backscatter received is due to somebody else on the internet doing something bad and spam-related.
Types of backscatter:
Misdirected bounces from spam runs, from mail servers who “accept then bounce” instead of rejecting mail.
Misdirected virus/worm “OMG your mail was infected!” email notifications from virus scanners.
Misdirected “please confirm your subscription” requests from mailing lists that allow email-based signup requests.
Out of office or vacation autoreplies and autoresponders.
Challenge requests from “Challenge/Response” anti-spam software. Maybe C/R software works great for you, but it generates significant backscatter to people you don't know.
How bad is this problem?
Spam lists contain a high percentage of invalid addresses, driving a high bounce rate. A normal mailing to a legitimate list will result in 3-10% of the mail bouncing, per mailing. And that's if you handle bounces properly. When senders haven't tracked bounces properly, a list can have anywhere from 30-50% bounce rate. Spammers rarely handle bounces properly, so let's assume a 40% bounce rate on a spam run. If a spammer sends two million pieces of spam, that leaves 800,000 bounces. Where do those bounces go? Maybe 7-10% of email servers accept the mail then bounce it back later.
Do the math based on all of these assumptions: 2,000,000 spams, 40% bounce rate, 9% of mail servers send backscatter... That means that for that two million spam run, 72,000 bounce notifications (NDRs) are going to be sent back to the sender address. Since spammers forge the sender's address, this mail is going to be be received by people who had nothing to do with the spam. This, in a nutshell, is backscatter. And there's a lot of it floating around.
How to stop it?
There's probably not a ton you can do to prevent the receipt of backscatter. Some anti-spam blacklists actively block servers that generate backscatter. Spamcop used to do so in the past, but they actively target these kind of servers at the moment. Though, any spamtrap-based DNSBL is going to (intentionally or not) catch servers sending backscatter, because the backscatter will hit their spamtrap addresses just like it's hitting you and everybody else.
Whatever you do, don't use a “Challenge/Response” anti-spam application or service. It makes the problem worse for everybody else on the internet – your challenge requests are just another kind of backscatter.
The same goes for those anti-spam applications that promise to send fake bounces on your behalf. The reasoning is that the spammers will realize your address is dead, and stop sending you mail. That's simply not true. You're going to bounce off the lists of good senders, who actually process bounces. The bad guys (whom are responsible for far more of the mail you receive) don't process bounces. They're the ones creating this whole backscatter problem to begin with. Yours will be just another bogus bounce notification bothering some innocent, unrelated third-party.
Don't set an “out of office” reply, either. Besides contributing to the backscatter problem yourself, you're sending random notes out to the world telling whoever receives the notice that you have a live email address and you'll be back soon to receive it. Guess what? You're confirming for spammers that your email address is valid! That means you're going to get even more spam. I see over 800 out of office replies in my spamtrap mailbox. If I were a sleazy spammer, it would be very easy to write a script to save those addresses in a file, and wait and see how many more I get today, tomorrow, next week, etc.
Forward one of these messages (forward as attachment in Groupwise) to me and I will try to set a filter for your account.
Wait a few days for the flurry to stop. Spammers usually give up on a non-productive address after a few days so this should also stop the delivery failures notices being generated by the bounce.