In my last post I blogged about greylisting, a well-known anti-spam technique for rejecting spam sent by botnets. When a mail server receives a an attempt to deliver mail from an IP address that’s never sent mail before, it rejects the message with a “soft fail” error which tells the sender to try again later. Real mail senders always retry, badly written spamware often doesn’t. I found that even though everyone knows about greylisting, about 2/3 of IPs don’t successfully retry.

Another theory about greylisting is that if you defer mail from a new IP, by the time the sender retries, if it’s sending spam it’ll have hit spamtraps and been added to blacklists. I recently realized that I have enough log data to check that theory, so I collected some statistics for the past week, which is as long as I keep logs about mail connections from blacklisted hosts. The IPs I greylisted broke down like this:

No retry and Retry too soon are senders that greylisting kept from sending anything, again, about 2/3 of mail. (My greylister requires that the sender wait at least a minute, since some spamware sends several messages a few seconds apart.)

The next two are senders that retried successfully and sent one message, or more than one message. (If a sender retries too soon, then retries again after more than a minute, it’s counted in one of those two categories.) Blacklisted means that when the IP retried, the IP was on one of the a blacklists I use, in nearly all cases Spamhaus Zen. The last line is IPs that retried successfully, but were blacklisted when they tried to send other messages later.

The 5.3% for Blacklisted probably overstates how much mail was caught by waiting to see if an IP was blacklisted. My logs don’t say whether the delivery attempt that was blacklisted was trying to deliver a message with the same To and From addresses, in which case it would have been delivered, or a different message, in which case it would just have been greylisted again. Spot checking shows IPs that were greylisted repeatedly, before appearing in a blacklist, which suggests that they were sending different messages.

Also, for the few IPs that were blacklisted later, they were generally blacklisted much later, hours or days later, far longer than any reasonable greylisting strategy would force mail to wait.

So greylisting still works, but it’s almost entirely because spamware doesn’t retry, not because it gets blacklisted.