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After recently reinstalling my company's Exchange server, I encountered a problem. When users sent messages to internal email addresses, the recipients received their correspondence. However, when users sent messages to external email addresses, the users received nondelivery reports (NDRs).

After a careful examination, I discovered that the Internet Mail Service (IMS) routing option Reroute incoming SMTP mail (required for POP3/IMAP4 support) wasn't enabled. Instead, the Do not reroute incoming SMTP mail option was enabled. POP3 and Internet Message Access Protocol 4 (IMAP4) clients use SMTP to transmit messages to Exchange. The IMS doesn't handle incoming messages from these clients unless you configure it to do so. Thus, you need to include the client's Internet domain in the routing table and specify that Exchange route inbound messages from this domain to the IMS. In my case, I had to add my company's Internet domain to the routing table and set it to be inbound.

To get to this routing option, follow these steps:

  1. Start Microsoft Exchange Administrator.
  2. Select the organization name.
  3. Drill down to the site name and the Configuration option.
  4. Select Connections.
  5. Choose Internet Mail Service (ServerName) in the right pane of Exchange Administrator.
  6. Select Properties on the toolbar icon or Alt+Enter on the keyboard.
  7. Go to the Routing tab.

After I enabled the Reroute incoming SMTP mail (required for POP3/IMAP4 support) option and restarted the IMS, users were once again able to successfully send messages to external email addresses.