A SMTP error usually means the sender is not authorized for the action it attempted.

In MailSlurp custom-domain setups, the most common cause is DNS misconfiguration (especially incorrect MX records).

What typically means

Depending on provider, often maps to one of these conditions:

  • authentication required but missing
  • sender not permitted for this route
  • domain routing records are incorrect
  • SMTP host intended for internal/private use is being used publicly

Fast triage checklist

  1. Verify your domain MX records.
  2. Confirm you are using the public inbound MX endpoint.
  3. Check SPF/DKIM/DMARC health for sender identity.
  4. Confirm app credentials and sender envelope are valid.

Critical MX record rule for custom domains

For inbound handling, use only the public inbound endpoint record for your region.

Example expected value (us-west-2):

If you accidentally publish a private/internal SMTP hostname, Gmail/Yahoo and other providers may reject with or related authorization errors.

Validate MX records with dig

Expected pattern:

Notes:

  • the trailing dot is valid DNS notation
  • avoid duplicate or conflicting MX records during migration
  • allow DNS propagation time before retesting

Secondary checks when MX looks correct

  • sender domain/auth alignment (SPF/DKIM/DMARC)
  • wrong SMTP host/port for outbound path
  • stale credentials in app config or secret store
  • misrouted subdomain inbox configuration

Useful tools:

If you use subdomains

If your inboxes are configured on a subdomain, ensure that subdomain has the correct MX/auth records and is fully onboarded for your account.

If records look correct but failures continue, contact support with:

  • full SMTP error message
  • sending domain/subdomain
  • sample timestamp and recipient
  • current MX/SPF/DKIM/DMARC outputs

Support: /support/contact/