I've had many users with this problem, and in fact it is usually due to
Norton.
The interesting part is that what makes Norton unhappy is badly formatted
messages. No white space between headers and body for one, and whole
message having no actual CR/LF's instead having unix line separators for
another. Both occur relatively often in junk mail, and both pretty reliably
make Norton choke. Initially we had the problem with Norton (and other)
virus/junk filters that put up localhost POP3 servers. Later on we have
seen it even when Norton is configured to be involved less obtrusively with
mail. These programs just really don't seem to have been written robustly
in accordance with Postel's Law for Interoperability (be conservative in
what you send, liberal in what you will receive.)
Various versions of Outlook have at various rev levels also been inclined to
hang when reading badly formatted messages. Current revs of Outlook 2003
seem able to process whatever garbage they read from a POP3 server with no
problems at all.
One other reason for timeouts seems to be use of the preview with HTML
messages that include images that must be downloaded. Apparently some
versions of Outlook pause when previewing till that is done and this pause
can be long enough for standard POP3 servers to time out. Turning off
preview has solved the problem in these cases.