The most obvious symptoms of this value being incorrectly set are;
a. Mail routes internally happily.
b. Inbound internet mail is working fine
c. Outbound internet mail is not working
d. Other externally visible services generated by the server (eg: http) are visible on web.
Some of the Domino server console error messages you may see if this is wrongly configured include;
- DNS Not Implemented
- Messages about Relaying being Denied or Relay Hosts
- No messages transferred to xxxxx via SMTP: SMTP Protocol Returned a Transient Error
Where is it?
The setting in question lives in the Server Configuration documents on the Router/SMTP and then Basics tabs. The actual name of the setting is: "Relay Host for messages leaving the Local Internet Domain".
What should it be set to?
This depends mightily on your configuration but if you're going through some sort of email passthru gateway, such as an anti-spam appliance, then it should be set to the IP address of that appliance. This is particularly true of hosted environments where the server itself may not be granted direct SMTP to the Internet rights on the firewall.
If your server is going directly to the internet, then this setting should be blank. We tried lots of values including 127.0.0.1 to self-reference, but it took a leap to think of blanking it out entirely.