What is a mail relay?

A mail relay is an SMTP server that receives email from one source and forwards it to another destination, acting as an intermediary in the delivery chain. It's like a postal sorting facility—mail comes in from local post offices and gets routed toward its final destination. Every ESP operates mail relays; they receive your campaign from their web interface or API and relay it to recipient mailbox providers.

Relays can be open (accepting mail from anyone—extremely dangerous and commonly exploited by spammers) or authenticated (requiring credentials before relaying). Modern email infrastructure exclusively uses authenticated relays. Services like SendGrid, Mailgun, Amazon SES, and Postmark are essentially sophisticated relay services with added features.

The relay is where authentication meets the road. When your relay sends email, the receiving server evaluates the relay's IP reputation, checks SPF alignment with your domain, and verifies DKIM signatures. Your choice of relay (ESP) directly impacts whether your messages reach the inbox. Running your own relay means inheriting all responsibility for that IP's reputation—a significant undertaking for serious senders.