Oh, I found out now. Sorry for wasting your time.

Oh, how do I set a group as implicit then?

In T10684#215611, @OrangeStar wrote:

I'd like to at least test this before deploying to production. I'd like to know if it was made with wikifarms in mind, or if it behaves just like a local emailblock, which can be trivially bypassed.

Should probably also enable $wgEchoPerUserBlacklist to block unwanted Echo notifications