The SELinux is the culprit! You need to enable the sending of emails from one of its Boolean values to allow you to send these emails.
It works at the Kernel level and is a MAC (Mandatory access control).
If you execute the command:
You will have a list of values. There is a value called:
I was off! In order to make certain changes in the permissions of some applications the SELinux “turned off” that possibility. When we discovered it, we found a way to solve it:
setsebool -P httpd_can_sendmail on
Thanks to “hedicode” for the fix.