Ask Slashdot: Is Reverse DNS a Worthy Standard For Fighting Spam?

drmartin66 makes it to the front page with this question: "Last weekend I installed a new spam filter server for a client, and enabled connection rejection if the sending server did not have a Reverse DNS record. Since then, I have had a number of emails rejected from regulator bodies that do not have a Reverse DNS record, and are refusing to have one created for their email server. What is your opinion of Reverse DNS records? Are they (or should they be) a standard, and required? Or are they useless for spam fighting?"

Comments

rDNS

[alphatel]

Like all things spam, marking the message as bad automatically is generally discouraged. If you simply increase the SCL value by some reasonable number, and continue to raise SCL based on other soft violations (like spamhaus, surbl, etc), you will rarely put good senders in the junk email folder, and very frequently be able to reject most spam content.

Re:Depends on how badly you want mail....

[Just Some Guy]

It's been a long time since I wrote up some spam-filtering instructions, but I'd still stand by most of my recommendations. In general, yes: just increase the spam score. I do have several litmus tests, though. If you fail one of these, I'm not accepting your mail:

• Your HELO has to send something that actually looks like a hostname. "server" doesn't work, and neither does "5626^^^". Rationale: a server this badly misconfigured is either a spambot or so horribly broken that I don't want to talk to it. I look at the output of this rule from my logs and I've literally never seen anything blocked that looked like it might have been legitimate.
• Don't send me my own hostname in the HELO. You're lying. The only reason to do this is to trick me into relaying for you.
• Don't send mail From: an unresolvable address. "someone@server" isn't a legitimate email address. Neither is "joe@nonexistent.example.com". If it would be impossible to send you a reply because the address you've given can't possibly be valid, I don't need to hear from you.
• I use zen.spamhaus.org, bl.spamcop.net, and b.barracudacentral.org to generate a likely spam score for incoming servers. If their combined score exceeds a certain threshold, I outright block email from that server. A server might accidentally end up on a blacklist, but it's unlikely that one would accidentally end up on more than one of those (in my opinion and experience) very conservative lists.

"Be liberal with what you accept" is a great idea to a point, but there are some things that correlate very strongly with spamminess. Back to the subject at hand: I don't think that lack of reverse DNS is one of those things.