The service code 503 simply implies that the server is unable to handle a request due to overloading or maintenance of the server. 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.

Here’s what happens when the browser tries to communicate with the webserver:

• The DNS (Domain Name System) looks up the IP address from the IP name of the web site;
• The browser opens an IP socket connection to that particular IP Address;
• It writes HTTP data stream through the socket;
• It then receives an HTTP data stream back from the server, which contains status codes. They are then analyzed.

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. During that time, it is expected that the experts are working on fixing the problem. To troubleshoot for this error, you can try the following, as per Microsoft Support:

• Check if all services are running;
• Ensure the services are running under the Local System account;
• Mount the mailbox store and the public folder stores;
• See if a registry key that exceeds 259 characters exists in the HKEY_CLASSES_ROOT registry hive;
• Check whether Group Policy object exists – it will prevent the MSExchangeIS service from initializing;
• Re-register the MDAC Components;
• Verify the permissions for the HKEY_CLASSES_ROOT registry key.

Most of the time, status code 503 translates to “server not available, please come back in X hours.” Often webmasters return 503 on purpose. It is widely used during scheduled maintenance and website upgrades. This code is used to tell search engine agents that the content is not available, but it would be in a couple of hours (you can set any timeframe), and they should come back and crawl it then. This way, webmasters make sure the web page will remain in the search engines’ index. It is also a technique used to take the load of the server during peak periods.

One of the things you can do to protect your site from the much-unwanted downtime is to monitor your server for free and test your website frequently. There are a lot of good practices to make your website and server work better, but there is nothing like an excellent remote monitoring service.