Gmail sometimes shows a server error with a message “Error 503 (Server Error)!!1“. This error generally occurs due to the server inability to handle the HTTP requests.
At Google servers, the error code returned while server downtime is Service Error -27, take a look at the below screenshot how it displays in browser window:
Gmail 503 Server error is a matter of server downtime and to fix this problem, you may try the HTML only Gmail, chances are it may work for some people.
If the HTML mode doesn’t work for you, all you can do now is wait, wait and wait till the Gmail servers are up again.