Communication problems with email can be caused by many factors.

Sometimes email may accidentally be redirected to another account. It’s possible that a mail forwarding loop was inadvertently set up. Or that an end user has an Exchange account that is not their preferred email address.

Email can also be blocked due to email quota limitations. Mail filters may also block email based parameters associated with certain email attachments. Bandwidth limitations can also be the cause of email download problems.

All administrators should be logging error messages. All email transactions can be logged and tracked by enabling logging in Outlook. This will allow an administrator to review the logs and identify undesirable trends or patterns with email. Logging of email transactions can be enabled using the following steps:

1. From the Tools menu select Options
2. Choose the Other tab.
3. Click on Advanced Options.
4. Verify that “Enable logging” is enabled.
5. Click OK

If the problem you are having is about long download times from an Outlook 2007 client and you are using a Post Office Protocol (POP3) email server then there are some additional possibilities.

Also, if you encounter time out errors when using a Simple Mail Transfer Protocol (SMTP) server you may get an error message such as:
 
“Task ‘Main – Sending and Receiving’ reported error (0×800CCC0F): The connection to the server was interrupted. If this problem continues, contact your server administrator or Internet service provider (ISP). “

If you receive this error message then it’s an indication that a network hardware device, such as a router, does not support TCP Window Scaling.

Transmission Control Protocol (TCP) packets include a “window” field in the header. This “window” field is used to specify how much data the system which sent the packet can receive from the other end. TCP uses this window setting to control the maximum amount of data that is being sent and received between two systems during the communication process. This window value prevents one system from sending more data than the receiving system can process.

Most large enterprise servers today have huge amounts of memory compared to servers a decade or two ago. All this memory allows the servers to buffer more network data than servers of used in the earlier days when TCP windows were relatively small. Nowadays network adapters used in the enterprise are of the four gigabits per second or eight gigabits per second variety. These network adapters can handle larger amounts of data and thus are able to support a larger TCP window size.

There is a Fixit available on the Microsoft web site, but you may want to first determine if the problem is being caused by a network hardware device that does not support TCP Window Scaling.

An administrator can make this determination by performing the following steps:

1. Click on Start and then click on All Programs
2. Next click Accessories.
3. From there right-click on Command Prompt and then choose “Run As” and select the Administrator account. You may be asked to type in a password or just click Continue.
4. Once you have the command prompt window up then type in the following command and then hit the enter key:
   netsh interface tcp set global autotuninglevel=disabled
5. Close the Command Prompt window.
6. Restart the computer.
7. Start Outlook.

To verify the settings you can use the command:
netsh interface tcp show global

You should output similar to the following:
TCP Global Parameters
Receive-Side Scaling State                 : enabled
Chimney Offload State                      : enabled
Receive Window Auto-Tuning Level           : disabled
Add-On Congestion Control Provider         : ctcp
ECN Capability                             : disabled
RFC 1323 Timestamps                        : disabled

If the problem is fixed, you can choose to keep the value for the autoTuningLevel parameter as disabled or you can perform further diagnostics on your network devices to determine which device does not support TCP Window Scaling.

If you wish to set the autoTuningLevel parameter to other values then your other choices include:

• disabled
  Sets the receive window at the default value.
• highlyrestricted
  Lets the receive window grow beyond the default value, but does so very conservatively.
• restricted
  Lets the receive window grow beyond the default value, but limits such growth in some scenarios.
• normal
  Lets the receive window grow to accommodate most scenarios.
• experimental
  Lets the receive window grow to accommodate extreme scenarios. This value may decrease performance in common scenarios and should only be used for research purposes.

Some of the network hardware that is possibly affected by TCP window scaling include: Routers, Virtual Private Network endpoints, and Firewalls that use Stateful Packet Inspection (SPI).