There are some functionality problems associated with the usage of Outlook 2007 on Windows Vista and POP3 accounts. Outlook 2007 users on Windows Vista have complained that the application becomes virtually unusable with POP3 accounts. The issue is limited to Outlook 2007; Outlook 2003 is not affected.
The issue is closely connected to a new feature in Windows Vista, namely TCP Windows Scaling. In the scenario in which a network hardware device, a router for example, does not deliver support for TCP Windows Scaling the users are going to experience the slow download speeds mentioned above.