Well, no - it had nothing to do with switching off POP, because Mail was back to misbehavin' a short while later. Now the truth is that I much prefer using Mail to Thunderbird, so I really would like to work this out. In case anybody is interested, here's a short video of what my mail activity window looks like (
http://tinyurl.com/6gnl8g). I've had IMAP enabled for about 10 months now, since whenever it was introduced, and the message that's being downloaded there was fully downloaded long ago - and several times over: I've seen it show up in the mail activity before now, along with other messages that I can confirm (with a Spotlight search) are already downloaded. Doesn't anyone have any ideas? I have removed the account and reinstated it more than once - and I have reinstalled Mail as well... But so far no joy...