A week or so ago I observed that the sole purpose of my secondary MXes seemed to be to send me spam a second time after I had rejected it the first time. So I removed all secondary MXes from my DNS zone. This resulted immediately in a lower load on my mail server, and no obvious ill effects.
Of course, the purpose of a secondary MX is to take over if and when my mail server goes down. But since most mail server will keep trying for several days anyway, it seems pretty unlikely that this is going to ever be a concern. So I’ll leave them off for now, and put them back on when I get ready to move, in case there’s an extended outage.
Meanwhile, my hard drive is spinning a lot less, and is thus much quieter.