Sorry if this is a duplicate. I tried to find this suggestion, since it felt pretty obvious to me but I only found this comment that mentioned it.
I'd like to see support for fully custom domains in Relay, instead of only being able to register a subdomain in mozmail.com.
- Custom domains might make it harder for sites to detect masks (although they could probably figure it out with a DNS lookup but that's much more expensive than a simple text filter).
- Having relays under a custom domain makes it easier to migrate between different providers.
It would make it easier to migrate away from Relay, but the same would apply to users that may wish to migrate to Relay from other providers that offer this feature (or a basic catch-all setup).
This also includes the potential (if far-fetched) possibility of Mozilla shutting down Relay. Without custom domains, recovering access to masked address would be impossible.