In rare circumstances an application may not be able to use traditional mail flow mechanisms to deliver mail and rely on another transport mechanism such as an installation of Microsoft SMTP. I ran into an issue recently where when a system was trying to use Microsoft SMTP to send a message it would deliver fine to internal users, but not to external users. It was a bit baffling as to why this wasn’t working until more research was done.
Research
- In order to properly research logon to the server where the Microsoft SMTP service is installed. This can be done by going to Start, All programs, Internet Information Services (IIS) 6.0 Manager
2. Right-click on [SMTP Virtual Server #1] and choose properties
3. Click on the Delivery Tab and then click the Advanced button in the bottom right-hand corner
Resolution
1. Make sure the Smart Host is configured to go through your external mailgateway. Example: mail.gateway.com
Note: In some cases the smart host entry will need to be your external relay connector. It will depend on your internal mail configuration and overall Exchange or Email system design.