STARTTLS
STARTTLS, more officially called Opportunistic TLS, is an extension to any unencrypted protocol that enables upgrading the connection to TLS.
Usage
Following the establishment of a TCP connection:
server sends 220 ready
client sends EHLO
server sends 250 STARTTLS (among other available commands)
client sends STARTTLS
server sends 220 GO HEAD
Now a TLS negotiation begins.