I’ve been using postgrey for a while now in order to cut down on my spam. However, a few days ago, I realised that I hadn’t also killed my backup MX. So the greylisting, whilst helping, wasn’t being as effective as it could.
A few days ago, I decided to get rid of the backup MX and have my own mailserver handle the load on it’s own. After all, it’s been pretty reliable and I didn’t see much need for it.
I only connected the two events yesterday when I suddenly realised that my spam volumes were far, far lower than usual. But it makes sense. Backup MX’s reduce the effectiveness of greylisting. Doh.
Thinking about why, it’s because greylisting particularly hurts those clients (like zombie spam engines) which don’t implement proper queueing for mail. But a backup MX is much more likely to use correctly implemented software (like exim in my case). So it’s quite happy to wait the requisite 5 minutes until the greylisting wears off.