Fetch only working with dummy email trigger

Hello!
I am trying to send send a push notification to my phone whenever there is an incoming email. For that I am using ntfy.sh, which uses a POST request with fetch, as shown in my code below.
This works perfectly when using the dummy Email Trigger Event, but it does not work with a real email.

When I send a real mail to the worker, it gets forwarded just as intended, but the fetch just does not work.

function sendNotification() {
  return fetch('https://ntfy.example.com/mail', {
    method: 'POST',
    body: "new mail",
    headers: {
      'Authorization': 'Bearer exampletoken'
    }
  });
}

export default {
  async email(message, env, ctx) {
    const block = ["[email protected]", "[email protected]"];

    if (block.indexOf(message.headers.get("from")) === -1) {

      await sendNotification();

      await message.forward("[email protected]");

    } else {
      message.setReject('Address is blocked');
      return 0;
    }
  }
};