olemus
tekstiedastusprotokollide standardlaiend, mis võimaldab luua krüpteeritud ühenduse otse TCP-ühenduse peal, kasutamata selleks eraldi porti
= a standard extension to text transfer protocols that allows an encrypted connection to be established directly on top of a TCP connection without using a separate port

ülevaateid
https://en.wikipedia.org/wiki/Opportunistic_TLS

https://certified-senders.org/wp-content/uploads/2020/02/Email-Transport-Encryption-STARTTLS-vs.-DANE-vs.-MTA-STS_updated.pdf

https://www.usenix.org/system/files/sec21-poddebniak.pdf

standardid
IMAP, POP3 -- https://www.ietf.org/rfc/rfc2595.txt
SMTP -- https://www.ietf.org/rfc/rfc3207.txt
FTP -- https://tools.ietf.org/html/rfc4217
XMPP -- https://tools.ietf.org/html/rfc6120
LDAP -- https://www.ietf.org/rfc/rfc2830.txt
NNTP -- https://tools.ietf.org/html/rfc4642