How to send emails through php so that they reach "Gmail for domain"?

Question

PHP

There was such a problem. We are using google api to work with mail on our domain. Letters have been successfully coming from the site to the post office for several years. Letters were generated by a php script, the mail function.
But in December last year, the letters stopped coming. I have no idea what to do. We haven't changed anything on the site. Whether any requirements of Google have changed is also unclear, I did not find any information about this.
Can someone suggest what to do?
It is worth noting that ordinary letters (not from the site) come to the mail and everything works well.

Answer 1

Set up sending mail via SMTP.

Answer 2

Something has changed on the site/server. Because this still works today (the mail function without any SMTP). Just look in your spam folder...

Answer 3

It seems to me that Souvel1 is right. Something has changed on the server, namely the mailer thinks that it sends letters to itself and processes them locally. Who administers the server?