Mail no longer working after upgrading to 1.5.9

If you can change the code, set the debug level on PHPMailer

In the file :
application/modules/mailer/helpers/phpmailer_helper.php

function phpmail_send(
    $from,
    $to,
    $subject,
    $message,
                 ...
                ...
    // Create the basic mailer object
    $mail = new \PHPMailer\PHPMailer\PHPMailer();
    $mail->CharSet = 'UTF-8';
    $mail->isHTML();
    
    $mail->SMTPDebug = 2;    // or 3 or 4