Current SMTP client implementation is not optimal. It acts like HTTP/1.0: one request per connection.
I'd like to see an SMTP client, which will have an SMTP transport by an analogy to http.Client and http.Transport.

Transport should allow to perform a RoundTripper method, reuse idle connections, have a map of connections per host, efficiently reuse established AUTH sessions to send messages within the ​same auth session when possible, etc.

P.S. I know this is a big task, and it will require a lot of effort. But at the end it will give a similar flexibility as net/http package give. I appreciate objections, feedback, suggestions.

Reference emersion/go-smtp-proxy#9