The SMTP service receives mails sent to the local domains by any server on the internet. The service, in turn, relays all received mails to the backend server, delivers them locally, expands it to a mail list, etc. It always runs on port 25, and its address must be published in a DNS MX record.
mx.js is the most strongly associated configuration file.
It configures the SMTP service, the filter chain, and most filter parameters.