Send a small controlled test through the same production path the campaign will use.

Check:

  1. visible From and Reply-To;
  2. SPF, DKIM and DMARC;
  3. links and redirect domains;
  4. plain-text and HTML rendering;
  5. unsubscribe behavior;
  6. List-Unsubscribe headers where applicable;
  7. MIME/attachment structure;
  8. actual SMTP source and bounce domain;
  9. message size;
  10. provider-specific requirements.

A template preview inside the marketing platform does not prove the delivered message has the right headers or authentication.

Reference material