The biggest problem I see here is forwarding someone else's unknown and unencrypted content. Imagine you're running a relay on your machine, and somebody posts some CSAM, or stolen CC data, or hate speech, etc through your relay. Unlike an email client, your relay actually makes that message available to anyone. Unlike an email relay, you are not forwarding the message to someone else like a dumb pipe; there's no fin…
Like email.
> Unlike an email client, your relay actually makes that message available to anyone.
On the contrary. Public mailing lists work exactly like this. You send a "subscribe" message (or enter your email address and click on a "subscribe" button, or whatever), and the server starts forwarding all the messages to you.