> If you keep the TLS connection open, sending an HTTP request is very fast. Waiting for the roundtrips of that chatter reduces the throughput, especially on higher latency links.
I agree that for a high load of sending email it can be beneficial to use an intermediate service. IMO a local sendmail is still preferred, but I see your point.
However, the Mailit service makes the client wait on the SMTP traffic too [1]. So with this particular service you now have to wait for both SMTP /and/ HTTP communication. Doesn't really speed thinks up :)
[1] https://github.com/dthree/mailit/blob/master/src/routes.js#L...