SMTP relay

Simple Mail Transfer Protocol (SMTP) relay, also called email relay, is the process of relaying messages from one server to the other using the SMPT protocol. SMTP relay helps to ensure that email messages are delivered quickly and reliably, even if the sender and recipient are using different email systems.

There are two main types of SMTP relay services: an open relay (or public SMTP relay) and a closed relay. Open relays send emails from external and internal sources, while closed relays only send internal server emails.