Here's some pointlessly complex systems administration stuff.
I spent an hour or two today debugging my spam filtering setup. Most of my e-mail goes through Caltech, which does spam tagging nicely, but recently there's been a substantial increase in e-mail coming through various hosted domains. This bypasses Caltech's tagging, so I get spam that comes that way. I only noticed when I started getting 100 to 150 spam messages a day, which seemed like a poor false-negative rate for spamassassin...
I'd set up my spamassassin+exim4 config according to Martijn Anthonissen's excellent Combining Exim4 with Spamassassin page. Unfortunately I pretty quickly figured out that none of my e-mail was getting processed by my setup -- so I finally took the time out to understand all of the different configuration stuff on that page.
Irony of ironies, it turns out that my e-mail was getting delivered to procmail in my .forward file before it was hitting the spam tagging system; forward files are ranked at 500 or so in exim, as opposed to the spam tagging system, which is ranked at 850. I changed the number to 450 and everything started working!
I'm not sure what Martijn does, but it seems a bit odd that you'd have a spam tagging system that only operated after forwardfiles; he must use something to filter tagged spam out of his box...?
In the process I discovered that my exim4 logs were over 200mb in size, and growing. A bit of investigation showed that logrotate was installed and configured to rotate the exim4 logs -- so why wasn't it happening? It turns out (boggle) that logrotate dies silently if one of the log files it's supposed to monitor doesn't exist.