May 14th, 2006

OK Computer

More anti-spam measures

Now that I'm running Sendmail 8.13 (thanks to Solaris 10), I'm able to add a new anti-spam feature called greet_pause.

This feature delays the initial SMTP greeting your mail server issues by a configurable amount (I'm using five seconds). If a client says anything before the greeting, the mail server doesn't accept any of its commands, and logs a message like this:

May 14 11:02:30 mailman sendmail[3896]: [ID 801593 mail.info] k4EF2PNm003896: rejecting commands from [218.62.77.50] [218.62.77.50] due to pre-greeting traffic

Like greylisting, this relies on the fact that lots of spammers aren't using well-behaved SMTP servers. Many of them are using simple programs that just blast all their SMTP commands at your mail server immediately, without trying to have an actual SMTP conversation. The greet_pause feature will reject mail from clients like those. And since those clients probably don't even check for failures, they won't even retry.