SMTP must maintain a transient state throughout the connection and respond to interactive requests...<p>HTTP-POST can handle the request chain inline, and deliver a single response, the decision tree is much lighter.<p>For that matter, HTTP-POST can partially to completely defer all processing decision trees, always returning 200 OK, offloading to a queue system that will process the request out of band, and issue the real response errors via a dashboard.  Making it lighter still.