TLS

QTLS alternative to STARTTLS omits server response before TLS handshake and adds server capability greeting after handshake.

save 2RTT

or client may pipeline STARTTLS command + TLS handshake
which requires interleaving SMTP and TLS data (tricky)
but works better with TLS session cache and greet-pause skip.

save 1RTT or 2RTT

(Server may offer any combination of these features)