add_action('wp_mail_failed', 'log_mailer_errors', 10, 1);
function log_mailer_errors($mailer){
  $fn = ABB_FRAMEWORK . 'mail.log'; // say you've got a mail.log file in your server root
  $fp = fopen($fn, 'a');
  fputs($fp, "Mailer Error: " . json_encode($mailer->get_error_messages()) ."\n");
  fclose($fp);
}

add_action( 'phpmailer_init', 'mailer_config', 10, 1);
function mailer_config(PHPMailer $mailer){
    $mailer->IsSMTP();
    $mailer->SMTPAuth = true;
    // $mailer->SMTPSecure = 'ssl';
    $mailer->Host = 'host';
    $mailer->Port = 465;
    $mailer->SMTPDebug = 2;
    $mailer->Username = 'xxxxx';
    $mailer->Password = 'xxxxx';
}