public async Task NoCertificateAvailable_ReturnsErrorResponse() { var mocks = new Mocks(); mocks.ServerBehaviour.Setup(b => b.GetSslCertificate(It.IsAny <IConnection>())).Returns <X509Certificate>(null); var verb = new StartTlsVerb(); await verb.ProcessAsync(mocks.Connection.Object, new SmtpCommand("STARTTLS")); mocks.VerifyWriteResponseAsync(StandardSmtpResponseCode.CommandNotImplemented); }