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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?
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By default most systems allow SSL 3.0, TLS 1.0, 1.2 and 1.2 to be used, when connecting using SMTP client. smtp.
While STARTTLS has TLS in its name, it's not necessary to use TLS; users can choose SSL instead. The difference between SSL/TLS and STARTTLS is the latter is not a protocol but a command issued between an email program and a server. STARTTLS notifies a mail server that the contents of an email need to be encrypted.
This class allows you to attach files, streams, or text to an email message. MailAddress. Represents the email address of the sender and recipients. MailMessage. Represents an email message.
TLS is the preferred encryption method because it's newer and offers more robust security features than SSL does. It's also a good idea to combine TLS-based email encryption with email authentication to ensure the integrity of email messages.
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.
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