Hello,
Addressing your questions in order:
Since you should be using https, messages will have to go through port 443.
When you visit the API endpoint via your browser, this is the message that you should expect to see.
When you open an IPN message via the CP interface, does it report as being sent? Or did sending failed?
If it failed, this is a very common issue that people face, usually hosts do not allow incoming messages by default. There could be firewalls preventing the messages either on your host machine or on your host’s network. You would have to speak with your server administrator about this.
best of luck
with regards