What is a smart host or relay host?

A smart host (or relay host) is a mail server configured to accept outbound messages and forward them to their destinations. Instead of your application or local server delivering directly to recipient mail servers, it hands off all mail to the smart host, which handles final delivery.

Common uses: centralizing outbound mail through an ESP or filtering service, applying consistent authentication and policies across multiple sending systems, routing mail through a server with better IP reputation, or complying with network policies that restrict direct SMTP connections.

Configuration involves pointing your local MTA's relay settings to the smart host, typically with authentication credentials. The smart host then becomes responsible for DNS lookups, connection handling, and delivery attempts.

Rather than every rowboat navigating the open sea, all cargo routes through the main vessel. Centralized routing simplifies management and standardizes the journey.