I finally had a chance to have another look at the IMAPd issue I was having, and I sort of fixed it.

I cheated a bit, I just backed up the one on the new disk, and copied the old one off the old disk, replacing the newer one.

I tried to run it on the command line to test it, and just got:

/usr/sbin/imapd: error while loading shared libraries: libc-client.so.4.7: cannot open shared object file: No such file or directory

so I found the library on the old disk, and copied it across to the new one (cp libc-client.so.4.7 /usr/lib), and then when I ran it again manually:

/usr/sbin/imapd
* OK localhost.localdomain IMAP4rev1 v12.264 server ready

Sweet. I tried to connect with thunderbird again, but it complained that I had secure authentication enabled, and the server didn't support it.

I must have left that on when I was mucking around trying to get it to work with the newer imapd a while ago.

I turned it off, and it connected straight away. It's probably a bit less secure, running that old imapd, but it's not accessible from the internet anyway, only over an ssh tunnel, so I don't see the problem.