Red Hat Sendmail FAQ
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3.7 How do I route all mail to unknown users to a single account?

Add this to your macro config file and rebuild sendmail.cf: 

   define(`LUSER_RELAY',`local:someuser@your.domain')dnl
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3.6 Sending mail to a user in my domain I get an error message. It says, “config error. mail loops back to me (MX problem?)”. How do I fix this?Up3.8 I need to act as a secondary MX for a friend of mine's domain, but inbound mail for that domain gets rejected. How do I make this work?