Your Server can support many independent Domains in addition to the Main Server Domain.
In order to process Messages and Signals directed to your Server Domains, you should ensure that the Messages sent to that domain are directed to your Server with the global DNS system.
Example 1: your server (example.com) serves the example.com Domain and a partners-example.com Domain. Make sure that DNS MX records are created for both Domains, and that those records point to your example.com Server.
Example 2: your server (example.com) acts as an "Remote POP" mail host relay for some client systems. Each client has its own domain name (client1.com, client2.com, and client3.com), and you have configured your Router to ensure that all E-mails sent to the client1.com domain will be routed to the Unified Domain-Wide Account client1, etc. You should also ensure that when an E-mail is sent to the client1.com domain, that E-mail is routed to your server (example.com). The client1.com MX record should be created in the DNS system and that record should point to your Server (example.com).