10.1 Update Mail Background Fetch Failed Port 110 Timed Out



The subject really says most of what is going wrong.

Since I've upgraded to OS X 10.1, the message fetching
process for Mail has become extremely unreliable with a POP host.
Very occasionally it succeeds, but most of the time, it just
spins and spins and eventually pops up a dialog box saying:

"Fetch Error

Background fetch failed with exception. The
connection to the server pop-gc.gu.edu.au on port
110 timed out."

I can reliably telnet to the POP server on port 110 and it's doing the right
sort of things:

# telnet pop-gc.gu.edu.au 110
Connected to redback.ins.gu.edu.au.
Escape character is '^]'.
+OK <13920221.1027@redback.ins.gu.edu.au>, MercuryP/NLM v1.48 ready.
telnet> quit
Connection closed.

so the problem is obviously happening with the software preferences
or networking (or something) on my machine.

Does anyone have any ideas about what might have changed or been
overlooked in the upgrade to 10.1 and/or how to fix it?