Simple Mail Transfer Protocol (SMTP) is a protocol used to send and receive email between mail servers.
Internal Self-Managed SMTP Server
In many networks, the email server, running via SMTP, exists internally to the protected network, usually on a demilitarized zone (DMZ). In such situations, it's common practice for the domain name servers (DNS) to have their mail exchange (MX) records for the appropriate domain to contain the IP address of the Smoothwall Firewall. The Smoothwall Firewall then uses a system of network translations to direct incoming email to the local SMTP server.
External Self-Managed SMTP Email Server
In many company infrastructures, the company email server exists on an external network. Either, at a remote geographical location, or a server in some form of network data center.
In SMTP email delivery, the sending email process attempts to register where to deliver the email. The server performs a specialized DNS request to find the address of the server that handles email for the appropriate domain because the email address is of the form [email protected]. This is known as an MX record.
A domain might have several MX records because mail servers are prone to being unavailable for periods of time, each given a numeral value. MX records are processed lowest numbered first by default. An email is delivered to whichever server responds correctly, starting with the one with the lowest number.
If the DNS MX record for example.com currently points at 18.104.22.168, it's necessary to break this arrangement and insert the SMTP relay before it reaches the server 22.214.171.124.