Smtp client = new Smtp();
client.ValidatingCertificate += client_ValidatingCertificate;
client.Connect("smtp.gmail.com", SslMode.Implicit);
// continue with the work here
static void client_ValidatingCertificate(object sender, SslCertificateValidationEventArgs e)
{
if (e.Certificate.Thumbprint == "correct thumbprint")
e.Accept();
else
e.Reject();
}