STARTTLS vs SSL/TLS [closed]

Question

Mailclients offer both settings, the STARTTLS and SSL/TLS. What is the difference between STARTTLS and SSL/TLS?

Answer 1

The clearest explanation that I've read is from FastMail:

SSL and TLS both provide a way to encrypt a communication channel between two computers (e.g. your computer and our server). TLS is the successor to SSL and the terms SSL and TLS are used interchangeably unless you're referring to a specific version of the protocol.

STARTTLS is a way to take an existing insecure connection, and upgrade it to a secure connection using SSL/TLS. Note that despite having TLS in the name, STARTTLS doesn't mean you have to use TLS, you can use SSL.

Read the rest of the info they have, including a discussion of port numbers and POP/IMAP vs SMTP, here: https://www.fastmail.fm/help/technology_ssl_vs_tls_starttls.html

Answer 2

The question is quite vague, but I guess I understand it. STARTTLS means "explicit TLS" where the connection is established on regular port and then STARTTLS command is sent to initiate SSL handshake and switch to protection mode. Another option probably defines implicit SSL/TLS on a dedicated port. In implicit mode first the handshake takes place and then the application-level protocol runs over the established secure channel.