Yesterday Emily's email stopped working.
She reads email on 10.5.6 OS X Mail (mail.app) which sync via IMAP to our Google Apps family domain Gmail service.
I could see things looked fine on the Google side, so I opened the Mail.app activity window and saw it was stuck on "updating cache directory" (Google search link).
It seems this is a fairly common OS X Mail.app bug, but everyone I read had a different "fix".
Mine was very simple. I assumed it was an OS X problem; Apple is notorious for cache issues. I did a "safe boot" (hold shift on restart). That clears out some caches and is the first thing I do for most OS X problems. I like to do a "safe boot" every few months anyway; it's a bit of superstition perhaps.
After the "safe boot" I restarted. I didn't login, I just restarted normally.
The problem was fixed.