What is an SMTP relay?

An SMTP relay is the delivery engine that accepts your message and forwards it to the next server. It focuses solely on transport. It does not manage templates, lists, or analytics. It is the long distance cargo vessel that handles the hauling while someone else manages the cargo manifest.

The distinction matters. An ESP is the full sending platform. An SMTP relay is only the outbound delivery mechanism. Developers often use transactional relays such as Postmark or SendGrid for A to P application sending while marketing teams use a full ESP such as Mailchimp or Cakemail for bulk campaigns.