When you use Outlook or Outlook Express to send an e-mail message that has an attachment, the Outlook or Outlook Express program may stop responding (hang) during the sending process.

WARNING: If you use Registry Editor incorrectly, you may cause serious problems that may require you to reinstall your operating system. Microsoft cannot guarantee that you can solve problems that result from using Registry Editor incorrectly. Use Registry Editor at your own risk. To resolve this behavior, change the registry value for the size of the transmission unit. Use the appropriate procedure for the kind of connection you use.

Windows XP: If You Connect by DSL, Cable Modem, or LAN

1.	Click Start, click Run, type regedit, and then press ENTER to start Registry Editor (Regedit.exe).
2.	Locate the 000n value, where n represents a number that is assigned to the network adapters, under the following key in the registry: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Class\{4D36E972-E325-11CE-BFC1-08002bE10318}
3.	Search through each of the listed 000n values to find the DriverDesc key that pertains to the Ethernet adapter.
4.	In that same 000n key, note the NetCfgInstanceId number.
5.	Now find that noted number under the registry key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces
6.	Double-click the MTU value, and then type 576" in the Value data box. (576 is the decimal value, 240 is the hex value.) NOTE: If the key does not exist, create it. To do so: a. on the Edit menu, point to New, and then click DWORD Value. b. Type MTU and press ENTER. c. Double-click MTU, type 576, and then click OK. NOTE: 576 is the decimal value, 240 is the hex value.
7.	Quit Registry Editor, and then restart the computer.

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Windows 98 Second Edition or Millennium Edition (Me): If You Connect by DSL, Cable Modem, or LAN

1.	Click Start, click Run, type regedit, and then press ENTER to start Registry Editor (Regedit.exe).
2.	Locate the 000n value, where n represents a number that is assigned to the network adapters, under the following key in the registry: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Class\Net\
3.	Search through each of the listed 000n values to find the DriverDesc key that pertains to the Ethernet adapter.
4.	Double-click the MaxMTU value, and then type 576 in the Value data box. NOTE: If the key does not exist, create it. To do so, on the Edit menu, point to New, and then click String Value. Type MaxMTU and press ENTER. Double-click the value, type 576, and then click OK.
5.	Quit Registry Editor, and then restart the computer.

Modem: If You Connect by modem

1.	Click Start, click Run, type regedit, and then press ENTER to start Registry Editor (Regedit.exe).
2.	Locate the 000n value, where n represents a number that is assigned to the dial-up adapters, under the following key in the registry: HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\Class\Net\
3.	Search through each of the listed 000n values to find the DriverDesc key that pertains to the dial-up adapter or modem.
4.	Double-click the IPMTU value, and then type 576 in the Value data box. NOTE: If the key does not exist, create it. To do so, on the Edit menu, point to New, and then click String Value. Type IPMTU, and then press ENTER. Double-click the IPMTU value, type 576 in the Value data box, and then click OK.
5.	Quit Registry Editor, and then restart the computer.

NOTE: If the issue continues to occur, repeat the preceding steps, but type a different MTU value between 1,454 and 576. When you test additional MTU values, try 576 and then increase the number to 600. If setting the MTU value to 600 does not resolve the issue, increase the value by 100 until you reach 1,400. If the issue continues to occur, try 1,454 and then stop. When you are finished, if the issue is still not resolved, change the MTU value back to 576.  The maximum transmission unit (MTU) setting controls the maximum size of a TCP/IP packet that your computer sends. If the Internet service provider has a limit for what can be received, your packets may become fragmented, thus causing problems.