Before integration, decide:

  1. visible From domain and address;
  2. Reply-To behavior;
  3. bounce/return-path domain;
  4. DKIM signing ownership;
  5. SMTP or API authentication;
  6. secret/token rotation;
  7. retry and duplicate handling;
  8. delivery-event processing;
  9. logging/correlation ID;
  10. suppression rules.

Separate transport success from business success

An SMTP 250 or API 202-style acceptance is not proof the user received or acted on the message. Model later delivery events separately.

Reference material