hello,

Using outlook via proxy server to connect to exchange 2013

Get a HTTP/1.1 404 Not Found from the exchange server with the following header

X-CasErrorCode: MailboxGuidWithDomainNotFound

Getting this header regardless of the authentication method set in Outlook Anywhere.

OWA access for the same user works fine.

Any suggestions what this issue is related to ?

Thank you.

What Outlook version and what's your Exchange 2013 version?

Have you tested using testexchangeconnectivity.com?

It sounds like Autodiscover is failing for some reason.