I imagine this domain will quickly end up on lists like this one: https://knowledge.hubspot.com/forms/what-domains-are-blocked... The real value here is the opening of the source code. Set up a cheap domain, set up a cheap VPS, use Tailscale or similar to keep the web UI private, then you're good.
You can simply register a domain on domains.google, and they give you email aliases with each domain. The trick is that while you are limited to 5 aliases, you can define the * alias and it will redirect any mail recieved at that domain. The mail then ends up in your mailbox, but you can easily block adresses that do too much while not breaking the workflow of recieving emails you want.