I received a task from a client to edit a form on his website. The form used the mail() function of PHP. I changed the 'to' address (first parameter) to his email address.
As soon as I changed the email address I started receiving the following error.
With a GMail address or another domain address (mine, for example) it worked and I received the email in a couple of minutes.
Code snippets are pointless as it is a simple php mail function.
He owns the server, he's hosting the website in house so I believe it may be a misconfiguration problem.