PHP mail() funktioniert nur noch sehr sporadisch

<?php
  function sendMailBase($to, $subject, $msg, $reply) {
    $header  = "MIME-Version: 1.0\r\n";
    $header .= "Content-type: text/html; charset=iso-8859-1\r\n";
     
    $header .= 'From: "ABC NAME" <A@B.de>' . "\r\n";
    $header .= 'Reply-To: "ABC NAME" <' . $reply . '>' . "\r\n";
    $header .= 'X-Mailer: PHP/' . phpversion() . "\r\n";
    $header .= 'Delivery-Date: ' . date('r');

    return mail($to, $$subject, $msg, $header);
  }

  if (sendMailBase('empfänger', 'Test-Betreff', 'Test-Nachricht', 'absender')) {
    echo 'sent';
  } else {
    echo 'not sent';
  }
?>

<?php
  // Import PHPMailer classes into the global namespace
  // These must be at the top of your script, not inside a function
use PHPMailer\PHPMailer\PHPMailer;
use PHPMailer\PHPMailer\SMTP;

// Load Composer's autoloader
require 'vendor/autoload.php';

  // Instantiation and passing `true` enables exceptions
  $mail = new PHPMailer(true);

  try {
      //Server settings
      $mail->SMTPDebug = SMTP::DEBUG_SERVER;                      // Enable verbose debug output
      $mail->isSMTP();                                            // Send using SMTP
      $mail->Host       = '';                       // Set the SMTP server to send through
      $mail->SMTPAuth   = true;                                   // Enable SMTP authentication
      $mail->Username   = '';                // SMTP username
      $mail->Password   = 'secret';                               // SMTP password
      $mail->SMTPSecure = PHPMailer::ENCRYPTION_STARTTLS;         // Enable TLS encryption; `PHPMailer::ENCRYPTION_SMTPS` encouraged
      $mail->Port       = 587;                                    // TCP port to connect to, use 465 for `PHPMailer::ENCRYPTION_SMTPS` above

      //Recipients
      $mail->setFrom('', 'Mailer');
      $mail->addAddress('', '');     // Add a recipient             // Name is optional
      $mail->addReplyTo('', 'Information');

      // Content
      $mail->isHTML(true);                                  // Set email format to HTML
      $mail->Subject = 'Here is the subject';
      $mail->Body    = 'This is the HTML message body <b>in bold!</b>';
      $mail->AltBody = 'This is the body in plain text for non-HTML mail clients';

      $mail->send();
      echo 'Message has been sent';
  } catch (Exception $e) {
      echo "Message could not be sent. Mailer Error: {$mail->ErrorInfo}";
  }
?>

function sendMailWithMailer($to, $subject, $body, $reply, $from_name) {
    use \PHPMailer\PHPMailer\PHPMailer;
    use \PHPMailer\PHPMailer\SMTP;

    // .. ab hier geht nichts mehr weiter

    if (file_exists('../../PHPMailer/src/PHPMailer.php')) {
      echo '1';
    } else {
      echo '2';
    }

    require '../../PHPMailer/src/PHPMailer.php';
    require '../../PHPMailer/src/SMTP.php'; // 2 Verzeichnisse zurück, da ich mich so gesehen in /data/mail befinde (der Aufrufer).
...