Back on the 13th I noticed acme's loadav was up to 8, and there was no idle CPU time. I don't know what was going on - maybe some new attack - but I decided it was time to switch off the old IP address and mail address. I've been running on the new address only for the past few days. You can see from the stats on the right that it has been very effective.
I'll probably re-enable the old address at various points in the future, to get a reading on the spam level. But basically, acme.com is dead! Long live mail.acme.com!