I'd like to have an option to set SmtpClient.SslCipherSuitesPolicy option for the message relay to only allow certain cipher-suites for hardening purposes.
This option is only supported on Linux with OpenSSL (see https://github.com/jstedfast/MailKit/blob/4777e7f907ea0283e5ed053068e6f08d0ed0eb01/MailKit/IMailService.cs#L91 and https://github.com/dotnet/runtime/blob/64c0dfbe02c7678f7294ffceb5f0f77f18ffa187/src/libraries/System.Net.Security/src/System/Net/Security/CipherSuitesPolicy.cs#L12).
It'll throw a PlatformNotSupportedException if a CipherSuitesPolicy is created on an unsupported platform, which I'll catch and only log a warning.

I'll open a PR as soon as I have this feature ready.