`masquerade_domains`

Address masquerading is a method to hide all hosts below a domain behind their mail gateway, and to make it appear as if the mail comes from the gateway itself, instead of from individual machines.

Address masquerading is disabled by default. To enable, specify one or more domain names separated by whitespace or commas. For example: `$mydomain` ; in this example, addresses of the form `user@host.$mydomain` would be rewritten to `user@$mydomain`.