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Dealing with backscatter, revisited

Back in January, I wrote about how I deal with email backscatter nowadays. Since then, I’ve made a notable tweak.

This is that I no longer reject “null-sender” traffic during the SMTP transaction. It turned out that it broke Exim’s implementation of Sender Address Verification, which performs the SAV check using a MAIL FROM of <>, rendering it indistinguishable from a bounce during the SMTP transaction.

Now, I’ve complained about SAV, but I have to be pragmatic anyway (Postel’s law and all that!) — so it was better to just allow other sites to perform SAV lookups against our server, and fix the anti-bounce stuff some other way.

The new method (below) does this, by allowing null-sender SMTP traffic just fine; it detects bounces in Postfix if they arrive via SMTP in RFC-3464 format, and bounces that slip past are then dealt with in a more CPU-intensive manner using the SpamAssassin “VBounce” ruleset (which is part of the now-released SpamAssassin 3.2.0, btw).

This increases the load, since some bounces cannot be rejected at MAIL FROM time now, and instead we have to wait ’til DATA — but CPU hasn’t been a problem recently, so this is ok.

Here are the updated instructions:

In Postfix

In my Postfix configuration, on the machine that acts as MX for my domains — edit ‘/etc/postfix/header_checks’, and add these lines:

/^Content-Type: multipart\/report; report-type=delivery-status\;/  REJECT no third-party DSNs
/^Content-Type: message\/delivery-status; /     REJECT no third-party DSNs

Edit ‘/etc/postfix/’, and ensure it contains:

header_checks = regexp:/etc/postfix/header_checks

Then run:

sudo /etc/init.d/postfix restart

This catches most of the bounces — RFC-3464-format Delivery-Status-Notification messages from other mail servers.

In SpamAssassin

As before, install the Virus-bounce ruleset and set it up. This will catch challenge-response mails, “out of office” noise, “virus scanner detected blah” crap, and bounce mails generated by really broken groupware MTAs — the stuff that gets past the Postfix front-line.