title: Validating SSL certificate behind firewall
body: <p>I'm connecting to 3rd party services using SSL certificates. My server is running behind firewall with blocked outgoing traffic. The application is not able to validate the 3rd party certificate.</p>
  <pre><code>MailKit.Security.SslHandshakeException: An error occurred while attempting to establish an SSL or TLS connection.
  
  The server's SSL certificate could not be validated for the following reasons:
  • The server certificate has the following errors:
    • The revocation function was unable to check revocation for the certificate.
    • The revocation function was unable to check revocation because the revocation server was offline.
  • An intermediate certificate has the following errors:
    • The revocation function was unable to check revocation for the certificate.
    • The revocation function was unable to check revocation because the revocation server was offline.
  </code></pre>
  <p>I was trying to google how the client is validating certificate chain, but haven't found any. I'm especially interested at which ports and to which addresses the application / server is doing its validation queries.</p>
  <p>As an immediate hotfix I had to allow plain HTTP outgoing traffic to all destinations.</p>