SMTP Relay
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An SMTP relay is an intermediary [[mail-server|mail server]] that accepts outbound email from an application and forwards it to the recipient's mail server on the sender's behalf. Transactional email services (SendGrid, Postmark, AWS SES) operate as SMTP relays, providing dedicated IPs with established sending reputation, delivery analytics, and bounce handling. Using an SMTP relay requires adding its IP ranges or include mechanism to your domain's [[spf-record|SPF record]], and configuring [[dkim|DKIM]] signing through the relay provider, to ensure authenticated delivery from your custom domain.
Example
A Django application uses SendGrid as an SMTP relay: outbound mail authenticates as [email protected], passes through SendGrid's high-reputation IPs, and arrives in inboxes rather than spam — with full open/click tracking in the dashboard.