# SMTP relay: Definition and How It Works

Source: https://wraps.dev/glossary/smtp-relay

InfrastructureSMTP gatewaySES SMTP interface

An SMTP relay accepts mail over the standard SMTP protocol and forwards it on, letting any system that only knows how to speak SMTP — not call an API — send through a provider like SES.

## Why SMTP still matters next to a modern API

A huge amount of software only ever learned to send mail one way: connect to a server, authenticate, and speak SMTP. WordPress plugins, legacy PHPMailer and Nodemailer setups, network appliances, and countless internal tools were never built to call a REST or SDK-based sending API, and rewriting all of them isn't realistic. An SMTP relay lets that software keep doing exactly what it already does while the mail underneath actually goes through modern infrastructure.

## SES's SMTP credentials aren't your IAM credentials

SES exposes its own SMTP interface with a dedicated username and password pair, generated through a specific derivation from an IAM secret key rather than being the IAM access key and secret themselves. This is a real, separate credential type from the ones used for the SESv2 API, and it's the one to hand to any system that only knows how to configure "SMTP host, port, username, password" and nothing else.

## The regional endpoint has to match where you verified

SES's SMTP endpoints are regional, matching the SESv2 API's own regional scoping — an SMTP relay pointed at the wrong region's endpoint will authenticate against different verified identities than the ones actually configured, producing the same kind of identity-not-verified failures documented on the \`/ses/errors\` pages, just reached through SMTP instead of the API.

## Everything downstream still applies

Mail sent through the SMTP relay is still subject to the exact same SPF/DKIM/DMARC requirements, sending quotas, and suppression-list enforcement as mail sent through the API — SMTP is a different door into the same building, not a separate mail system with different rules.

## Why it matters

SMTP relay access is frequently the deciding factor in whether a legacy system, a plugin ecosystem like WordPress, or an appliance that can't be reprogrammed can move onto SES at all — without it, migrating those systems would mean either rewriting software that has no reasonable rewrite path or leaving it on older infrastructure indefinitely.

## Common questions

### Are SES SMTP credentials the same as my AWS access keys?

No. SMTP credentials are a separate username and password derived specifically for the SMTP interface, distinct from the IAM access key and secret used for the SESv2 API. Each is scoped and generated differently.

### Does mail sent through SMTP get the same deliverability treatment as API-sent mail?

Yes. SPF/DKIM/DMARC checks, sending quotas, and suppression-list enforcement all apply identically regardless of whether a message arrived through the SMTP relay or the API — SMTP is just a different entry point into the same sending pipeline.

### Why would I use SMTP relay instead of the SDK if I'm writing new code?

You generally wouldn't — SMTP relay exists for software that can only be configured with host/port/username/password and can't call an API or SDK at all, like most WordPress plugins or legacy mail libraries. New code should use the API or SDK directly.