Diagnosis.

[camwyn]

It does, indeed, appear to have been that our IP address was on a spamlist. Or at least on a list of open relays, which amounts to the same thing. The problem seems to have been fixed for the moment. I've contacted our ISP to ask them to do something about this so that we don't run into more crud like this in future; in the meantime, the mail's working again. I wish we didn't have to use that damn router/firewall/whatever, I get the feeling our provider doesn't really pay much attention to it... oh well. Their problem.