I have come across lots of people having this problem and believing it was
due to Outlook when it's actually due to a Vista setting relative to using
one of server types listed above. I put it up here to get more visability.
I pulled this from another site's newsgroup and it worked like a charm for us.

solution:

If you are using a pop3 or imap server, here is a fix:
start/Accessories/r-click on Command prompt and select "Run as
administrator" and then type: netsh interface tcp set global
autotuninglevel=disabled and then hit "Enter" key and it should say "ok". I
would restart pc just to be sure.

I know of at least 5 people/org in which this has fixed the issue.