Does Mailkit SMTP support TLS 1.3?
This is a short and sweet question, but in my googling for an answer I couldn't find a definitive response. Further I couldn't find a simple test email server, or service, where I could lock down the protocols such that only TLS1.3 was supported so I could do a simple test. And I had to add this paragraph because SO didn't think my one line question was of sufficient quality.
No, MailKit does not support TLSv1.3... but only because the .NET class libraries do not support TLSv1.3.
Once .NET supports TLSv1.3, so will MailKit.
To control which versions of the SSL and/or TLS protocol versions that you want a MailKit client to use, you can set the SslProtocols property to any of System.Security.Authentication.SslProtocols values available for your version of .NET.
Note: It seems that .NET 4.8 will support TLSv1.3