There's a few ways to approach this question. One is to mention community networks (DIY ISPs) which will ensure you always have a public IP without filtering. S

There's a few ways to approach this question. One is to mention community networks (DIY ISPs) which will ensure you always have a public IP without filtering. Some even provide VPN access so that you can use your filtered internet to acquire a publicly-routable IP. This is a common pattern in the ffdn.org federation of non-profit ISPs. Another one is to mention hosting coops (libreho.st/chatons.org) and how they could be employed in limited-network situations. On the web, we have SNI/eSNI-aware proxying which enables multiple servers to share a single IP without revealing their private keys to the reverse proxy. I don't know of an equivalent in the email world (because it's assumed there is only one MX with a canonical domain/DKIM per IP), but i'm all ears if you have suggestions! Of course, we could mention onionMX and other key-routing systems (CJDNS..) but the problem is you need it to be supported on the other side as well, which is highly unlikely.