Another problem that is caused by new Windows Vista feature – TCP Windows Scaling. The symptom for Outlook 2007 slow problem is that when you download email messages from a POP3 mail server, the Microsoft Office Outllok 2007 may take a very long time to download the e-mail message. For example, for a relatively small 1-megabyte (MB) in size email message, the download process may take one hour.

Microsoft KB935400 blames the cause of the Outlook 2007 slowness to network hardware device, such as a router, that does not support TCP Window Scaling. However, the problem does not exist in Windows XP, thus the problem is probably more due to poor implementation of TCP Windows Scaling feature, or at least due to this feature that does not provide backward compatibility with older network equipment.

To solve the problem, simply disable the Windows Vista AutoTuning function which is the most common suggestion and fix for various network problems in Windows Vista.

Here’s the step to disable TCP Window Scaling Auto Turning feature:

  1. Click Start button, click All Programs, and then click Accessories.
  2. Right-click Command Prompt, and then click Run as Administrator.

    If you are prompted for an administrator password or for confirmation, type the password, or click “Continue”.

  3. At the command prompt, type the following command, and then press ENTER:
    netsh interface tcp set global autotuninglevel=disabled
  4. Try to download the emails with Outlook 2007 again. The slow downloading problem should have resolved. Microsoft also suggest users to isolate the hardware that does not support TCP Window Scaling, replace the hardware and re-enable the autoTuningLevel parameter.