SSL and TLS terms are often used in confusing ways:
- SSL (Secure Sockets Layer) is the original protocol implementation. SSLv3 is still allowed by Dovecot, but it's rarely used. Some clients use SSL to mean that they're going to connect to the imaps (993), pop3s (995) or smtps (465) port, although they're still going to use TLSv1 protocol.
- TLS (Transport Layer Security) replaced the SSL protocol. TLSv1 protocol is used practically always nowadays. Some clients use TLS to mean that they're going to use STARTTLS command after connecting to the standard imap (143), pop3 (110) or smtp port (25/587). Nothing would prevent using SSLv3 protocol after STARTTLS command.
Unfortunately there doesn't seem to be any clear and simple way to refer to these two meanings.
SSL term is much more widely understood than TLS, so Dovecot configuration and this documentation only talks about SSL when in fact it means both SSL/TLS.
SSL works pretty much the same universally, so for more information about SSL you can see for example Apache's documentation.
- Dovecot uses OpenSSL, so whatever information you find about it applies also to Dovecot.