Verify the permissions for the HKEY_CLASSES_ROOT registry key.
Check whether Group Policy object exists – it will prevent the MSExchangeIS service from initializing.See if a registry key that exceeds 259 characters exists in the HKEY_CLASSES_ROOT registry hive.Mount the mailbox store and the public folder stores.Ensure the services are running under the Local System account.To troubleshoot for this error, you can try the following, as per Microsoft Support: During that time, it is expected that the experts are working on fixing the problem. The 503 status code occurs in the last step described above – the server is functioning at a minimum, meaning it does show the status code 503, but the website is completely unavailable. It then receives an HTTP data stream back from the server, which contains status codes.It writes HTTP data stream through the socket.The browser opens an IP socket connection to that particular IP Address.The DNS (Domain Name System) looks up the IP address from the IP name of the web site.Here’s what happens when the browser tries to communicate with the webserver: It is important to note that although this condition is temporary and simply causes some delay, some servers may refuse the socket connection, which will result in a different 5xx error code. The service code 503 simply implies that the server is unable to handle a request due to overloading or maintenance of the server.