Issue: Drupal cannot send email. Stack is on LAMP, distro is CentOS.
Cause: SELinux is blocking emails being sent out. To confirm, sestatus -b | grep sendmail.
httpd_can_sendmail is set to ‘off’, then that’s the cause.
Do NOT just turn SELinux off. Run
setsebool httpd_can_sendmail=1 to allow emails to be sent out.