XIMS: No Inbound SMTP Mail Because of NAT to Exchange Server (193863)

SYMPTOMS

Your Microsoft Exchange Server computer is set up behind a dial-up router connected to the Internet. The server can send SMTP messages out to the Internet, but cannot receive mail through the Internet Mail Service. Using Telnet to connect to the Exchange Server computer fails with a time out error message as well as pings to the Exchange Server computer. The Domain Name System (DNS) MX and A records appear to be correct.

CAUSE

Some routers are capable of doing Network Address Translation (NAT). In this scenario, the router is translating the Exchange Server computer's IP address to the router's IP address, thereby sending all mail on behalf of the Exchange Server computer, similar to the concept of our Microsoft Proxy Server. In addition, the router is accepting communication on behalf of the Exchange Server computer, but not allowing direct communication with Exchange Server. Neither trying to Telnet or ping the actual internal IP address of the Exchange Server computer, nor sending SMTP mail directly to Exchange Server, is possible.

WORKAROUND

To work around this problem, perform one of the following:

Option 1

Disable the NAT functionality on the router. This will allow direct communication to the Exchange Server computer from the Internet.
Update Internet Domain Name System (DNS) entries to reflect the actual IP address of the Exchange Server computer.

NOTE: The IP address referenced must be a valid InterNic assigned address.

Option 2

Change the Internet Domain Name System (DNS) entries to reflect the IP address of the router doing NAT.