My router at home is acting up now. All the email trouble I'd been having previously started happening again about a week ago, as evinced by an uncharacteristic lack of mail arriving. Some judicious probing of the SMTP server with telnet suggested that it was not spam blocking after all, but in fact trouble with the port 25 forwarding going awry. For some reason the router was sending traffic on that port to my Windows 2000 machine even though that's not the way it is configured. The Windows machine then shows firewall activity which I usually ignore (the indicator blinks all the time). The acid test is that when I power down the Windows 2000 box, all that pent-up mail starts flowing to where it ought to once more. (I could probably just unplug its CAT5 cable instead.) The odd thing is that this was not happening before, so something's changed in the network somehow, perhaps when we had all the DSL trouble at the time.
The last time this happened, it went away as mysteriously as it arose. I'll try power cycling things more or less at random to see whether I can get everything into the good state again.