You can configure your Simple Mail Transfer Protocol (SMTP) server to support multiple domains in order to host ISP functions. For the SMTP server to host ISP functions, it is necessary for SMTP to appear to operate in multiple domains. The SMTP client uses this configuration information to know which interface to bind to when it sends the e-mail, and which mail to consider local (resolve and send on its own) or to forward to a configured firewall mail daemon.
Related concepts
Prerequisites for an e-mail router