Thanks for the examples, if I specify tls1.2 (force) and the receiving mail server does not support it. Will it downgrade it to 1.1 or 1.0? Or just error out?
Exception will be thrown.
You can specify multiple protocols as SslProtocols is a flag:
client.SSLConfiguration.EnabledSslProtocols = SslProtocols.Tls12 | SslProtocols.Tls11;