That was weird

[sraun]

My mail server was have DNS issues. It was resolving a bunch of domains incorrectly - for instance, gmail.com to 1.0.0.0. There were several domains resolving to 1.0.0.0. I suspect there were some domains resolving to other incorrect addresses - yahoo.com, for one - and there's an indication that it's been happening for some time, slowly getting worse.

I think I've got it fixed now. I changed the resolve.conf - explicitly added my ISP's primary DNS server, and rebooted the box :-(, and rebooted the DSL device.

Unfortunately, I don't actually know what broke! This was something of a shot-gun fix - keep trying different things, some simultaneously.