Remember how Mail was driving me nuts?
I finally figured out what was going on. Two things that make IMAP an unhappy bunny: clock drift (yes, our server was about 30 minutes behind) and a missing list of subscribed folders. The former I fixed for now (must install a time sync package). The latter was fixed by doing the following on the server:
find Mail > .mailboxlist
… and then by editing out any incorrect concatenated directory names.
Now email is working fine, it’s fast, and better yet, I’ve finally been able to successfully synchronise the local and remote mailboxes so I have a copy of all my mail at this end. Hurrah!