Port 25 is used for email servers. This is because the server instance on a machine can prove to be run by an administrator. Additionally most providers don’t allow you to use port 25. Both techniques are used in order to avoid spam.
But: Chatmail is not vulnerable to spam. So couldn’t we just choose another port for a “Chatmail network”? It would be much easier to create your own server. You wouldn’t have to check all providers for allowing port 25 and check what you have to do to open it someday.
You could just take ANY provider or host it at home.