So for the past few weeks, I’ve been getting a lot less spam — like about 1/3 to 1/4 of the normal volume — to my jmason.org account.
I didn’t have a clue why; occasionally I mused that some spam gangs must have figured out that I needed all that spam to develop SpamAssassin, and cutting down on my volume would mean that I’d have to schlep stuff out of the spamtraps (which is a bit of a chore), so they’d unsubscribed me to cause some minor hassle ;)
In reality, what had happened was that my old secondary MX — which was secondarying for me because nobody had gotten around to updating it — had finally been updated, and was no longer accepting mail for jmason.org. So I had only one MX, and the erstwhile backup was bouncing anything it saw, immediately.
Lots of spamtools relay spam via the secondary MX — not sure why, we think it’s working on the assumption that secondaries are less likely to have effective filters.
So basically a good 2/3 to 3/4 of my spam was being sent to a machine that immediately bounced it ;)
The upshot: if you get a lot of spam, and don’t really care if you might occasionally lose real mail if your primary MXes are down, you could always set up a ‘fake’ secondary MX record. The spamtools will happily attempt to send spam to you via that machine (which may not even exist), and then give up after the first bounce — missing you entirely.
Big caveat: I wouldn’t suggest this for situations where your mail delivery needs to be reliable, though. Primary MXes do go down occasionally ;)