php' ) ; require( ' mailserver/ class. Here is the solution: http : / / www. marathon- studios. com/ blog/ php- mail- 503- error/. If you want to send responses with a freestyle status line, you need to use the ` header( ) ` function:. 500 = > " Internal Server Error", 501 = > " Not Implemented", 502 = > " Bad Gateway", 503 = > " Service Unavailable", 504 = > " Gateway Timeout",. A 404 error code means the requested item cannot be found at the location and no redirect was given.