PHP Mailer SMTP versender leere Emails

Hallo, ich benötige Hilfe beim Email versand. Es wird stets eine leere Email ohne Betreff gesendet:

Output of communication with SMTP server:
2024-05-08 11:46:12 SERVER -> CLIENT: 220 kundenserver.de (mreue012) Nemesis ESMTP Service ready
2024-05-08 11:46:12 CLIENT -> SERVER: EHLO kalendra.net
2024-05-08 11:46:12 SERVER -> CLIENT: 250-kundenserver.de Hello kalendra.net [82.165.87.195]250-8BITMIME250-SIZE 140000000250 STARTTLS
2024-05-08 11:46:12 CLIENT -> SERVER: STARTTLS
2024-05-08 11:46:12 SERVER -> CLIENT: 220 OK
2024-05-08 11:46:12 CLIENT -> SERVER: EHLO kalendra.net
2024-05-08 11:46:12 SERVER -> CLIENT: 250-kundenserver.de Hello kalendra.net [82.165.87.195]250-8BITMIME250-AUTH LOGIN PLAIN250 SIZE 140000000
2024-05-08 11:46:12 CLIENT -> SERVER: AUTH LOGIN
2024-05-08 11:46:12 SERVER -> CLIENT: 334 VXNlcm5hbWU6
2024-05-08 11:46:12 CLIENT -> SERVER: [credentials hidden]
2024-05-08 11:46:12 SERVER -> CLIENT: 334 UGFzc3dvcmQ6
2024-05-08 11:46:12 CLIENT -> SERVER: [credentials hidden]
2024-05-08 11:46:12 SERVER -> CLIENT: 235 Authentication succeeded
2024-05-08 11:46:12 CLIENT -> SERVER: MAIL FROM:
2024-05-08 11:46:12 SERVER -> CLIENT: 250 Requested mail action okay, completed
2024-05-08 11:46:12 CLIENT -> SERVER: RCPT TO:
2024-05-08 11:46:12 SERVER -> CLIENT: 250 OK
2024-05-08 11:46:12 CLIENT -> SERVER: DATA
2024-05-08 11:46:12 SERVER -> CLIENT: 354 Start mail input; end with .
2024-05-08 11:46:12 CLIENT -> SERVER: Date: Wed, 8 May 2024 11:46:12 +0000
2024-05-08 11:46:12 CLIENT -> SERVER: To: ruben2024@gmx.de
2024-05-08 11:46:12 CLIENT -> SERVER: From: KALENDRA
2024-05-08 11:46:12 CLIENT -> SERVER: Reply-To: Kalendra
2024-05-08 11:46:12 CLIENT -> SERVER: Subject:
2024-05-08 11:46:12 CLIENT -> SERVER: Message-ID:
2024-05-08 11:46:12 CLIENT -> SERVER: X-Mailer: PHPMailer 6.9.1 (https://github.com/PHPMailer/PHPMailer)
2024-05-08 11:46:12 CLIENT -> SERVER: sender: info@kalendra.net
2024-05-08 11:46:12 CLIENT -> SERVER: MIME-Version: 1.0
2024-05-08 11:46:12 CLIENT -> SERVER: Content-Type: text/plain; format=flowed; delsp=yes; charset=UTF-8
2024-05-08 11:46:12 CLIENT -> SERVER: Content-Transfer-Encoding: 8Bit
2024-05-08 11:46:12 CLIENT -> SERVER:
2024-05-08 11:46:12 CLIENT -> SERVER:
2024-05-08 11:46:12 CLIENT -> SERVER:
2024-05-08 11:46:12 CLIENT -> SERVER: .
2024-05-08 11:46:12 SERVER -> CLIENT: 250 Requested mail action okay, completed: id=1MuF4v-1suAXZ1KhR-00ua7l
2024-05-08 11:46:12 CLIENT -> SERVER: QUIT
2024-05-08 11:46:12 SERVER -> CLIENT: 221 kundenserver.de Service closing transmission chann

Mittwoch, 8. Mai 2024 - 11:46
Benutzer Gast (nicht überprüft)
Standort https://kalendra.net/registerpartnersave
Referrer https://kalendra.net/partner/register
Nachricht Sending email with body: Hallo Ruben, Thank you for registering at Kalendra.

email = $user->getEmail();
$firstname = $user->get('field_user_firstname')->value;

$body = "Hallo $firstname, \n\n";
$body .= "Thank you for registering at Kalendra.";

\Drupal::logger('contentsave_manager')->notice('Sending email with body: ' . $body);

// Konfiguration und Senden der E-Mail
$message = [
'subject' => '

Deine Registrierung bei Kalendra

',
'body' => $body,
'headers' => [
'Content-Type' => 'text/html; charset=UTF-8'
]
];

$module = 'contentsave_manager';
$key = 'registration_email22';

$result = \Drupal::service('plugin.manager.mail')->mail($module, $key, $email, NULL, $message, NULL);

if ($result['result'] !== true) {
\Drupal::logger('my_module')->error('Failed to send email. Error: ' . print_r($result, true));
return "Failed to send email.";
}
\Drupal::logger('contensave_manager')->notice('Sending email with details: ' . print_r($message, true));

return "ok";
}

vielen Dank für eure Hilfe