AuthenticationException when tryign to connect to mail server using MailKit

Question

I am using MailKit to try and send email though an exchange server via SMTP but when I try to connect I get the following error:

An exception of type 'System.Security.Authentication.AuthenticationException' occurred in MailKit.dll but was not handled in user code

Additional information: The remote certificate is invalid according to the validation procedure.

Googleing this brings up a lot of stuff about Gmail or as part of other error messages (like FTP or web api requests and such). I have talked with the IT guys and it is not a self-signed cert and we don't require authentication (in fact the BugZilla instance I admin is setup with the same settings and works fine). What am I doing wrong or how can I get more details to further troubleshoot?

using (var Client = new SmtpClient())
{
    Client.Connect("mail.address.com", 587, false);
    Client.AuthenticationMechanisms.Remove("XOAUTH2");
    Client.Send(Message);
    Client.Disconnect(true);
}

Edit: I have verified with IT that it is the same cert used in IIS that hosts the mail.address.com domain as well as in exchange. I have also installed it as a trusted root but still get the same error.

Edit 2: If I update the code to Client.Connect("mail.address.com", 587, true); then I get the error:

An exception of type 'System.IO.IOException' occurred in >System.Private.Networking.dll but was not handled in user code

Additional information: The handshake failed due to an unexpected packet format.

Answer 1

Try using Client.Connect("mail.address.com", 587, SecureSocketOptions.None); to disable STARTTLS.

If you want to keep STARTTLS, you might try overriding Client.ServerCertificateValidationCallback.

The easiest way to get more information about the error is to override ServerCertificateValidationCallback with something like this:

bool ValidateRemoteCertificate (object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
{
    Console.WriteLine ("SslPolicyErrors: {0}", sslPolicyErrors);

    return sslPolicyErrors == SslPolicyErrors.None;
}