Fight Spam With Nolisting

An anonymous reader writes with the technique of Nolisting, which fights spam by specifying a primary MX that is always unavailable. The page is an extensive FAQ and how-to guide that addressed the objections I immediately came up with. From the article: "It has been observed that when a domain has both a primary (high priority, low number) and a secondary (low priority, high number) MX record configured in DNS, overall SMTP connections will decrease when the primary MX is unavailable. This decrease is unexpected because RFC 2821 (Simple Mail Transfer Protocol) specifies that a client MUST try and retry each MX address in order, and SHOULD try at least two addresses. It turns out that nearly all violators of this specification exist for the purpose of sending spam or viruses. Nolisting takes advantage of this behavior by configuring a domain's primary MX record to use an IP address that does not have an active service listening on SMTP port 25. RFC-compliant clients will retry delivery to the secondary MX, which is configured to serve the role normally performed by the primary MX)."

Comments

That's "greylisting".

[khasim]

"Greylisting" is where an SMTP server refuses messages for a certain amount of time. You set the criteria on why the message would be refused and how long the server would refuse to accept it.

It's been pretty much defeated now because so many spammers have their machines try to hammer the message through until it does go through.

I'm using greylisting right now and the only advantage is that many times a spammer will end up on an RBL during the 15 minutes that I'm refusing his messages.

Remember, the spammers have, effectively, unlimted bandwidth and unlimited processing power at their disposal.