How does a sending API differ from SMTP submission?

A sending API uses HTTP/REST to submit emails—you POST JSON data to an endpoint, and the ESP handles everything. SMTP submission uses the traditional SMTP protocol—your application opens a socket connection, authenticates, and speaks SMTP commands. APIs are modern and simpler; SMTP is older but universal.

API advantages: easier integration (standard HTTP libraries work), better error handling (JSON responses with detailed codes), built-in features (automatic encoding, attachment handling), and typically faster development. SMTP advantages: universal compatibility (every email library speaks SMTP), works when APIs are unavailable, no vendor lock-in in the protocol itself.

Choose based on your use case. Transactional emails from web applications almost always use APIs—they're faster to implement and maintain. Legacy systems or applications behind strict firewalls might need SMTP. Bulk marketing typically uses the ESP's web interface or API batch endpoints. Most developers today never touch SMTP directly; APIs abstract the complexity while providing better observability through webhooks and detailed responses.