Does System.Net.Mail.SmtpClient use SSL, TLS, StartTLS, or a combination?

Question

This question has answers about negotiating the best version of TLS/SSL available with the server: Which versions of SSL/TLS does System.Net.WebRequest support?

However I thought SmtpClient used regular Smtp for transport, with TLS on top - which as I understand it is StartTLS.

I am documenting some software (a box ticking exercise) and need to state which protocols are used, and security protocols are really not my area of expertise so how can I explain this well?

Equalitarian

Answer 1

According to the documentation it can only do explicit TLS (i.e. STARTTLS) and not implicit TLS (SMTPS):

The SmtpClient class only supports the SMTP Service Extension for Secure SMTP over Transport Layer Security as defined in RFC 3207. In this mode, the SMTP session begins on an unencrypted channel, then a STARTTLS command is issued by the client to the server to switch to secure communication using SSL. See RFC 3207 published by the Internet Engineering Task Force (IETF) for more information.

Cystocarp