In Thunderbird (and I assume in many other clients, too) I have the option to choose between "SSL/TLS" and "STARTTLS".
As far as I understand it, "STARTTLS" means in simple words "encrypt if both ends support TLS, o ...
This section discusses the differences between SSL / TLS / STARTTLS. There is a lot of people on the internet that get all of these terms confused so this is a collection of links that will help explain the differences.

serverfault.com/questions/523804/is-starttls-more-safe-than-...
In Thunderbird (and I assume in many other clients, too) I have the option to choose between "SSL/TLS" and "STARTTLS".
As far as I understand it, "STARTTLS" means in simple words "encrypt if both ends support TLS, o ...

luxsci.com/blog/smtp-tls-all-about-secure-email-delivery-ove...
TLS, Transport Layer Security, seamlessly ensures transport email encryption for email delivery over SMTP between supporting servers. This is an easy to read article explaining SMTP TLS.

novosial.org/openssl/tls-name/index.html
This page outlines terminology problems with the Secure Sockets Layer (SSL) protocol, the Transport Layer Security (TLS) protocol, the term STARTTLS, and the Transmission Control Protocol (TCP) ports used for encrypted connections. Other information on ...

www.fastmail.fm/help/technology_ssl_vs_tls_starttls.html
There's often quite a bit of confusion about the different terms SSL vs TLS vs STARTTLS. This article explains the differences.

www.limilabs.com/blog/ssl-vs-tls-vs-starttls-stls
There's often quite a confusion about the different terms: SSL, TLS, STARTTLS and STLS. SSL and TLS SSL and TLS are cryptographic protocols, both provide a

stackoverflow.com/questions/5540374/starttls-vs-ssl-tls
Mailclients offer both settings, the STARTTLS and SSL/TLS. What is the difference between STARTTLS and SSL/TLS?
This thread gives an ecellent explanation of this subject.

superuser.com/questions/620745/thunderbird-ssl-tls-vs-startt...
Following up this question on serverfault "Is STARTTLS more safe than TLS/SSL?" it came up that the question is rather a question about Thunderbird and not about the actual protocols. Also, if you Google about this topic you will find ...