POP3 clients cannot connect to an Exchange Server computer that is behind an ISA Server firewall

Important This article contains information about how to modify the registry. Make sure to back up the registry before you modify it. Make sure that you know how to restore the registry if a problem occurs. For more information about how to back up, restore, and modify the registry, click the following article number to view the article in the Microsoft Knowledge Base:When a Post Office Protocol 3 (POP3) client tries to connect to a Microsoft Exchange Server computer that is behind a Microsoft Internet Security and Acceleration (ISA) Server firewall, the firewall may block the incoming POP3 connection.

Note After several attempts, the client may succeed in connecting to the Exchange Server computer.

This issue occurs if all the available Winsock listening sockets that accept incoming POP3 connections are allocated by the ISA Server firewall for incoming POP3 connections.

Warning Serious problems might occur if you modify the registry incorrectly by using Registry Editor or by using another method. These problems might require that you reinstall your operating system. Microsoft cannot guarantee that these problems can be solved. Modify the registry at your own risk.To resolve this issue , follow these steps:

1. Use any text editor, such as Notepad, to create a text file that has a .reg extension. Add the following text to the file:
   Windows Registry Editor Version 5.00
   
   [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Fwsrv\Parameters] "ServerMappingBacklog"=dword:00000032
2. Save the file.
   
   Note Make sure that you do not use the default .txt extension when you save the file. Instead, specify the .reg extension.
3. Double-click the .reg file to import the registry entry into the registry.

Note The accepted value for the ServerMappingBacklog registry entry is from 10 to 200, in decimal format.

If you still experience this issue after you follow these steps, increase the value of the ServerMappingBacklog registry entry.

To do this, follow these steps:

1. Click Start, click Run, type regedit, and then click OK.
2. Locate and then click the following registry subkey:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Fwsrv\Parameters
3. Right-click the ServerMappingBacklog registry entry, click Modify, and then click Decimal.
4. In the Value data box, type an appropriate value. For example, if the existing value is 32, increase the value to 50.
   
   .
5. Click OK.
6. Quit Registry Editor, and then restart the computer.