Show or link email domain blocklist state during account signup moderation

Pitch

Add an option for a built in check to show temp or privacy domain status in the account moderation UI, or a link out to a good third-party service.

Motivation

As with everyone else, we've got loads of spambots at the moment. In order to separate out the real and fake accounts, we check the mail domains against https://verifymail.io and automatically reject temp mail domains. But this involves copy/paste (just like IP addresses #40713) and it would be really useful to have this check built in somehow.

If it's built in, it would be amazing to have a rule to auto-reject somehow based on temp mail domains, though I realise there aren't really mail domain matching rules at the moment in the same way as for usernames and IPs.

As with #40713, as a third-party service would be required, this would have to be opt-in.