If you are using an imap (or apparently pop as well) server to send/receive email under Vista, here is a fix to speed up slow send and receive:
1. Open an elevated command prompt: Start->Accessories->Right Click on
Command prompt and select "Run as administrator"
2. Once command prompt opens type:
netsh interface tcp set global autotuninglevel=disabled (enter)
3. it should say "ok".
4. Close command prompt and restart the computer.
In my situation after reboot the first time using windows mail I was disappointed as it seemed slow. But I closed it and restarted and WOW it was SO much faster!! Tried it on a second laptop as well.
This disables some of the new TCP packet size features in vista that seem to be incompatible with the rest of the universe. I haven't noticed any adverse effets.