IPN from PayPal doesn’t work

I discovered today the IPN simulator in the PayPal Dashboard.

I tried several types:
Cart checkout
Web Accept
eCheck – Complete

All with the default parameters, to see, if the hook registration works and will be called.

The behaviour:
eCheck – Complete: Hook registration and hook call:
IPN was sent and the handshake was verified.
Cart checkout: No hook registration and no call, but:
IPN was sent and the handshake was verified.
Web Accept: Hook registration and hook call:
IPN was sent and the handshake was verified.

A simple Node.js script calling the IPN URL with some data works, to register the hook and call it:

const https = require('https')

const data = JSON.stringify({
  todo: 'Buy the milk'
})

const options = {
  hostname: 'my-domain.de',
  port: 443,
  path: '/?rest_route=/paypalipn/v1/cf7pp_production',
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
    'Content-Length': data.length
  }
}

const req = https.request(options, res => {
  console.log(<code>statusCode: ${res.statusCode}</code>)

  res.on('data', d => {
    process.stdout.write(d)
  })
})

req.on('error', error => {
  console.error(error)
})

req.write(data)
req.end()

Why does the simulator work for Cart checkout / eCheck and even a POST request script, but not for a real transaction with PayPal?

It was a security setting in the interface of my webhoster, which prevented the IPNs.
DANG!
See: https://www.webdecker.de/artikel/items/strato-paypal-ipn-http-error-503.html
So, if you have the same behaviour search in Google for: PayPal IPN 503
Maybe your hoster has a security switch, too.