NOTE: you can use both an ICANN domain and raw authenticated IPv6 with self-hosted email - going to the same mailboxes.
Also note that TLS is also a scam. Any CA listed by the shadowy TLS cabal can forge any domain for any browser using the default list of totally trusted for absolutely everything CAs provided by said cabal. Solution? Learn about PKCS#11 policies to NOT trust the cabal CAs for private TLDs (or any domain signed by private CAs). Trust private CAs only for associated TLDs or domains. (This is how it used to work before the cabal.)