I collected spam for 24 hours [1]. 40% of the 275 spams I collected were delivered through my backup MX (Mail eXchange) server, which was somewhat disapointing as I was expecting a larger percentage of spammers to use that techique.
But no matter.
I've gone ahead and changed the backup MX records to point to 127.0.0.1. I'm relunctant to leave it that way for very long (what happens if my primary MX server goes down?) but it will make for an interesting experiment for the next day or so.
And I hope to give a few spammers some headaches.