Glad it was helpful! I was worried I’d be a little off-topic talking about self-hosting e-mail instead of this Anon Addy thing. Hope you find a solution that works for you soon :).
And yeah… Unfortunately if you you’re behind CGNAT and don’t have a static IP I think doing this for free on your existing internet connection might be challenging. One thing that people in a similar position might be interested in is Hurricane Electric’s free Tunnelbroker service, but I think you might still be out of luck behind CGNAT.
You’ll be able to get public IPv6 addresses for free and can allocate them to your home network. You can set it up to dynamically update the IPv4 address on your end… But I think if you’re behind CGNAT you can’t do that, unfortunately. Another problem with this approach for something like a mail server is that not everything speaks IPv6… If a sender only supports IPv4 they won’t be able to send mail to you.
I think behind CGNAT pretty much your only option is to pay somebody for a real IP somewhere. Either a VPS somewhere where you set up wireguard (there are cheap options for this, and then you can run other things on the machine), or a VPN with a dedicated IP.