On some networks, Windows Vista may not allow you to download large email messages or file attachments to a POP3 account.
According to Microsoft, the issue is closely connected to a new feature namely TCP Windows Scaling. In the scenario in which a network hardware device, a router for example, does not deliver support for it the users are going to experience the slow download speeds mentioned above
Using Outlook 2007 or Windows Mail you get a message that the server has timed out, even if other e-mail accounts are working and making a test with telnet doesn't give you errors.
Until Microsoft releases a patch or fix for this issue, you can disable autotuning.
To Disable Autotuning in Windows Vista:
1) From the Start menu, select All Programs > Accessories.
2) In the Accessories list, right-click on Command Prompt and select Run as Administrator.
3) At the command prompt, type netsh interface tcp set global autotuninglevel=disable and press Enter.
4) When the command prompt returns confirmation of OK, close the command prompt window.