Plowing through the support forums, I can tell I’m not the only one having problems with this.
I have about 8 e-mail addresses in Mail. 5 of them are IMAP (4 from hosted domains, 1 from .MAC) and 3 are POP accounts. Since the upgrade, there’s been a slow and steady wasting and now, finally, completely death of my IMAP accounts. On the first day or so, everything worked fine, then I began experiencing painful delays sending and receiving, finally complete time-outs and now, nothing at all. (The .MAC IMAP account still works fine.)
What’s weirder, Mail seems to be reconfiguring my IMAP settings on the fly. The port numbers are different almost every time I go in the settings.
Using the one remaining computer that I haven’t upgraded to Leopard, everything still works fine.
I hope they fix this soon. I don’t like having to use Thunderbird for my e-mail. (Which also works just fine.)