Unofficial (Microsoft Internet Information Services)
the user's mailbox could not be reached

Specification of the HTTP status code 451

If the client is attempting to connect to the wrong server (that is, a server that cannot access the user's mailbox), or if there is a more efficient server to use to reach the user's mailbox, then a 451 Redirect Redirect error is returned.

Source / Quote by: The 451 Redirect HTTP Status Code is specified by section of HTTP IIS.

How to throw a 451 statuscode with PHP?

To throw the HTTP status code 451 on a web page, the PHP function http_response_code can be used. The syntax is as follows: http_response_code(451) (PHP 5 >= 5.4.0, PHP 7, PHP 8)

Test the 451 HTTP status code

In order to be able to display the HTTP status code (in this case 451 Redirect) and other information on the client side, the development console must be opened with F12. Afterwards you have to navigate to the tab "Network". Now you can open the page, in the network tab you should see the web page (example index.php). This must be selected and then the Herder section must be selected. Here the user will see the following result:

Status code 451 Redirect
URL: https://http-statuscode.com/errorCodeExample.php?code=451
Status: 451 Redirect
Those: Network
IP address: XX.XX.XX.XX
Status code 451 Redirect

Browser compatibility of the 451 status code

Chrome no data
Edge no data
Firefox no data
Opera no data
Safari no data
Chrome Android no data
Firefox for Android no data
Opera Android no data
Safari on iOS no data
Internet no data
WebView Android no data

Constants in programming languages

Author: Tony Brüser
Author: Tony Brüser

Tony Brüser is an enthusiastic web developer with a penchant for HTTP status codes.