What is an SMTP relay service?

An SMTP relay service is a third-party service that accepts outbound email from your application or mail server and handles delivery to recipients. Instead of your server connecting directly to every recipient's mail server, you hand off messages to the relay, which manages the complex delivery process.

The relay service maintains the infrastructure: IP addresses with established reputation, relationships with mailbox providers, bounce handling, and compliance with sending best practices. Popular relay services include SendGrid, Mailgun, Amazon SES, and Postmark.

To use a relay, you configure your application or mail server to authenticate with the relay's SMTP endpoint (typically on port 587 or 465). For example, instead of sending directly, your app connects to smtp.relay-service.com with your API credentials, and the relay takes over from there.

Think of it as hiring a shipping company. You prepare the packages and drop them at the depot; the shipper handles the trucks, routes, and last-mile delivery.