During my recent upgrade, I moved to Vista and Outlook 2007.  I have to say, I LOVE the Outlook 2007 UI.  I'm much more productive, and it introduces some very, very nice features, like overlapping calendars and integrated searching (although I still prefer X1, since it is MUCH faster and has a better UI). 

Still, I ran into this problem...  I couldn't use it to connect to my email server hosted on Windows Server 2003.  The email downloads were WAY to slow.  Retrieving 15 emails with a total of 100 kb took several minutes, and if I had an attachment larger that 1.2 MB I physically couldn't download it, since it would time out every time (after 30 minutes or so).  Yuck.

It was so bad that we gave up hosting our email on our own servers and found a service that would host all our email addresses, giving us POP3, internet access to our email, calendar and many other features free.  I looked all over for a solution to the slowness problem, including contacting Microsoft.  No one could help.  So we moved, and are quite happy with the new service.

Anyway, I've found the solution.  It wasn't Outlook 2007, but Vista, and it's a very common problem (based on Live and Google searches), without a well publicized solution.  It's the same solution as the last post. 

Go to the command line as an Administrator (in your Vista client) and execute the following command:

netsh interface tcp set global autotuninglevel=disable

Works like a champ!  Hopefully, this post will help a few people before they end up having to also migrate off of Windows Server 2003 to another solution. 

PS.  This problem doesn't affect all Windows Server 2003 (Win2K3) installs, only some.  Evidently it is a hardware issue.

UPDATE:  More information here:  Outlook is slow, RSS Broken from Tim Anderson's blog.  See comments 60-64 for more information and the solution presented here.