Outlook is unable to connect to the proxy server. (Error Code 0)

Scenario:  Users using Microsoft Outlook receive a pop up saying that Outlook is unable to connect to the proxy server. The exact error is:

There is a problem with the proxy server's security certificate. The name on this security certificate is invalid or does not match the name of the target site mail.domain.com.  

Outlook is unable to connect to the proxy server. (Error Code 0) 

Resolution:  We noticed that the Certificate Principal Name had a invalid value in the Outlook Profile.  In our case it showed a '-' in the field for 'Only connect to proxy servers that have this principal name in their certificate:'.  When we ran this command-let in Exchange Shell: Get-Outlook Provider, we saw there was a '-' for the Server and CertPrincipalName property.  This was causing autodiscover to hand this value out to Outlook Clients.  We resolved by resetting these values to $null:

Set-OutlookProvider EXPR  -server $null -CertPrincipalName $null