# SMTP relay

Authenticate with your API key prefix as the username and the full key as the password. The relay applies the same workspace checks, suppression, rate limits and tracking rewrites as the REST API — it is the same pipeline behind a different door, not a lesser path.

```text
Host:     smtp.mailhaap.com
Ports:    587   STARTTLS (required)
          465   implicit TLS
          2587  fallback, for networks that block 587
Username: the API key prefix, e.g. mh_live_a1b2c3
Password: the full API key
```

Three custom headers are honoured and stripped before delivery, so they never reach the recipient: X-MH-Tag, X-MH-Campaign-Id and X-MH-Track-Opens.

> **There is no open relay, ever** — Submission requires authentication without exception. Automated relay tests run hourly against our own submission hosts and alert on any anonymous acceptance.