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Sender Address Verification considered harmful

(as an anti-spam technique, at least.)

Sender-address verification, also known as callback verification, is a technique to verify that mail is being sent with a valid envelope-sender return address. It is supported by Exim and Postfix, among others.

Some view this as a useful anti-spam technique. In my opinion, it’s not.

Spam/anti-spam is an adversarial “game”. Whenever you’re considering anti-spam techniques, it’s important to bear in mind game theory, and the possible countermeasures that spammers will respond with. Before SAV became prevalent, spam was often sent using entirely fake sender data; hence the initial attractiveness of SAV. Once SAV became worth evading, the spammers needed to find “real” sender addresses to evade it. And where’s the obvious place to find real addresses? On the list of target addresses they’re spamming!

Since the spam is now sent using forged sender addresses of “real” people, when a spam bounces (as much of it does), the bounce will be sent back not to an entirely fake address, but to a spam recipient’s address.

Hence, the spam recipients now get twice as much mail from each spam run — spam aimed at them, and bounce blowback from hundreds of spams aimed at others, forged to appear to be from them.

This is the obvious “next move” in response to SAV, which is one reason why we never implemented something like it in SpamAssassin.

On top of this — it doesn’t work well enough anymore. Verizon use SAV. Have you ever heard anyone talk about how great Verizon’s spam filtering is? Didn’t think so.

(This post is a little late, given that SAV has been used for years now, but better late than never ;)

By the way, it’s worth noting that it’s still marginally acceptable to use SAV as a general email acceptance policy for your site — ie. as a way to assert that you’re not going to accept mail from people who won’t accept mail to the envelope sender address used to deliver it. Just don’t be fooled into thinking it’s helping the spam problem, or is helping anyone else but yourself.

Finally, this Sender Address Verification is different from what Sendio calls Sender Address Verification. That’s just challenge-response, which is crap for an entirely different, and much worse, set of reasons.