A couple of days ago, the SMTP logs on one of my machines lit up with a whole bunch of attempted inbound connections from all over the world. The first striking thing about these connection attempts is that they all seemed to be from people's home machines (what would once have been called 'dialups' but now uses cable modems, DSL, and various other technologies). Many of these machines were on the PBL and a couple that I just checked now are currently on the CBL.

The second striking thing is the interesting way that the spammer behind this snatched defeat from the jaws of potential victory. A few of these IP addresses actually got to talk to my SMTP daemon; when they did, they all reacted like this:

remote from [94.174.75.128]
HELO tvbtzzg.virginm.net
554 Unresolvable HELO name: tvbtzzg.virginm.net

remote from [172.10.0.198]
HELO koridl.sbcglobal.net
554 Unresolvable HELO name: koridl.sbcglobal.net

That's right. The spammer's software carefully worked out what the proper top level domain name was for the particular IP being used, put it in the HELO, and then made up a random hostname to go with it.

Given my usual views that spammers are by and large not stupid and are highly motived to do what works, I suspect that such HELO names must help their spam get through at least some spam filters (or, to put it another way, that other HELO names increase the risks of the spam email being filtered out). That very small operations like mine can use this to immediately reject their spam is presumably unimportant.

(I don't have any idea what would cause a spammer to think that my particular machine was worth turning a corner of a botnet on, instead of just using a compromised machine or two and then moving on. Perhaps it's a very big botnet. It seems to have moved on now.)