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Zimbra SMTP Relay

Use this guide when Zimbra should hand outbound mail to Mailtarget instead of delivering directly.

Mailtarget settings​

Relay host: smtp.mtrgt.net
Port: 587
Encryption: STARTTLS
Username: smtp_mt_injection
Password: <api_key with Send via SMTP permission>

Configuration outline​

  1. Create a Mailtarget API key with Send via SMTP permission.
  2. Verify the sending domain used by Zimbra users.
  3. Configure Zimbra's outbound relay host as smtp.mtrgt.net:587.
  4. Enable SMTP authentication and STARTTLS.
  5. Set smtp_mt_injection as the username and the API key as the password.
  6. Send a test message to an external mailbox.
  7. Confirm the delivery outcome in Mailtarget.

Checks​

  • If authentication fails, verify the username is exactly smtp_mt_injection.
  • If sender verification fails, check the domain in the From address.
  • If TLS fails, verify that the client is using STARTTLS on port 587, not implicit TLS.

Refer to the Zimbra version-specific admin documentation before changing production mail routing.