Allow WordPress to use a relay smtp with self signed cert

    271     /**
    272      * Options array passed to stream_context_create when connecting via SMTP.
    273      * @var array
    274      */
    275     public $SMTPOptions = array(
    276          'ssl' => array(
    277         'verify_peer' => false,
    278         'verify_peer_name' => false,
    279         'allow_self_signed' => true
    280                     )
    281         );

Around line 271 on wp-includes/class-phpmailer.php ,or better search for SMTPOptions

Related with same conf for Moodle as php now disables it.

Also phpmailer is a very popular class for php apps, in the documentation page on the troubleshot category you can find my case of use https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting#updating-ca-certificates