For anti-spam, the general use case is use of Tailscale/Zerotier where you are in control of your network. If you are on public internet and have 7777 open then you can use the Squelch filter under the CFG page. It drops every message that doesn't start with your secret::
> It drops every message that doesn't start with your secret::
Depending on how internet-proof you want to make this, I wonder if it might be better to sign with a secret and attach the signature to the message instead of directly sending the secret.
