There is a problem with the proxy server's security certificate.
Outlook is unable to connect to the proxy server mail.domain.com (Error Code 80000000).
The error was only experienced after moving a mailbox from Exchange 2010 to Exchange 2016. The issue was also intermittent, not all workstations received the error message.
On the Exchange 2016 server the following was configured:
- A valid digital certificate from RapidSSL was installed on the Exchange 2016 server along with the intermediate certificate and root certificate.
- All names for Internal and External URLs were set to a name that matched the digital certificate for Outlook Anywhere, MAPI over HTTPS, Exchange Web Services, OAB Distribution and Autodiscover.
- DNS was configured to direct all connections to the new Exchange 2016 servers.
- We had a proxy exclusion in place on all workstations to ensure data to the Exchange server "mail.domain.com" was sent direct, not through the proxy.
The server was configured with MAPI over HTTPS enabled on the organization configuration and all Outlook clients were creating MAPI over HTTPS connections to the server.
After much troubleshooting, we set the InternalClientsRequireSSL to $true and did an "iisreset" on the servers Outlook Anywhere configuration.
After changing InternalClientsRequreSsl to $true the issue did not occur.