XCON: Configuring the Internet Mail Service to Send and Receive Mail Through a Demand Dial Router

You may run into an issue when your connection to the Internet is through a demand dial router and you are using the Microsoft Exchange Server's Internet Mail Service. The Internet Mail Service forwards a message just as if it were a permanent connection. When the router detects that data needs to be sent, it initiates the dial, and sends the message. Exchange Server cannot tell the difference between this type of connection and a permanent connection, such as a T1 or Frame Relay. Most likely, when using a demand dial router, the primary MX record is your Internet server provider's (ISP's) mail server. The ISP queues the mail for the final destination. Normally, Exchange Server issues the ETRN command to the ISP's mail server to de-queue the mail. However, there is no way, within the Exchange Server Administrator program, to enable ETRN functionality outside of the normal Internet Mail Service dial-up configuration.

You can tell Exchange Server to always issue the ETRN command to the host specified in the Forward all mail to host field by setting the following registry parameter.

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.

1. Start Registry Editor (Regedt32.exe).
2. Locate the following key in the registry:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIMC\ Parameters
3. On the Edit menu, click Add Value, and then add the following registry value:
   Value Name: AlwaysUseETRN
   Data Type: REG_DWORD
   Value: 0x1
   
   
4. Quit Registry Editor.

Now, every time the Internet Mail Service forwards mail to the specified host, it will also issue the ETRN command to that host. There is no way to issue the ETRN command to another host. So, for this to work properly, the host that is queuing the mail also has to be able to relay mail.

One possible drawback to this approach is that incoming mail is only received when outgoing mail is sent. If this delay is an issue, you can configure an Exchange Server link monitor to the Internet. The link monitor uses an SMTP custom recipient that is a false user at a valid domain. The monitor sends a message to this custom recipient at the specified interval. Because this is an SMTP message, the Internet Mail Service sends it and also de-queues mail at the same time. Assuming the connection is functional, the message is returned as non-deliverable, indicating that the link is active. For the link monitor to stay active, the Administrator program needs to remain open. For additional information on configuring a link monitor, please see the Microsoft Exchange Server 5.5 Administrator's Guide.

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