[Enhancement] Provide a way to verify a namespace of e-mail addresses.

# Feature Request

## Description / Use Case

As was originally explained at [`webapps.stackexchange.com/q/39813`](https://webapps.stackexchange.com/q/39813/237205), users of e-mail alias services, of which AnonAddy and Firefox Relay and pertinent examples, are nigh unable to utilise Gravatar, because they must add a new address for every service that they register for:

Being able to have any e-mail address of the `beedellRoke.addy.IO` domain (and some additional [sub-]domains) be verified as owned by me would allow me to utilise Gravatar again.

## Proposed Solution

I propose that RegEx be permitted. It would require that services, like Firefox Relay and AnonAddy, provide a verification process that is compatible with Gravatar, likely bound to the user's sub-domain:

However, Mastodon and MetaMask already demonstrate that this should be feasible. [^1]

[^1]: [`joinmastodon.org/verification`](https://joinmastodon.org/verification)

## Alternatives

An alternative would be to allow a user to solely verify their ownership of a domain, then allow any e-mail address of that domain to verify a specific user.

This is less versatile, not least because it would not be useful to any organisation that does not provide a subdomain per user (which is most). However, it would be more verifiable.

## Additional Information

This relates to what I have previously requested, at [`nextcloud/contacts/issues/353`](https://github.com/nextcloud/contacts/issues/3530#issue-1816825315).

No source files, tests, or entry points are named in the issue. Start by asking maintainers which verification flow and service boundary should own namespace or domain verification; done would require an agreed design, implementation, and tests for the supported ownership model.

Written by the indexing model from the issue text.