Do you want a TLS-enabled SMTP *server* or do you want a *client*?
If you're after a server:
- you could use stunnel4 to use boring old SSL-tunnelled plain SMTP
- you could write a generic_ssl_server.erl module, by analogy with
generic_tcp_server.erl, which you could use in smtp_server:start/2
to get an stunnel4-like effect without requiring a separate wrapper
- you could implement STARTTLS in smtp_server_session.erl. This is
by far the trickiest of the options :-)