Rewrote DefaultServerCertificateValidationCallback
Rewrote DefaultServerCertificateValidationCallback

Instead of allowing all self-signed certificates, it will now
only accept common mail server certifictes in cases where the
root certificate is unknown/untrusted by the system (typically
due to a misconfigured system that has not imported the Root CA
certificates used by these mail servers).

Partial fix for issue #1002

jstedfast/MailKit,jamie-dainton/MailKit