Admin Panel / People / Domains

Every e-mail domain in use across the instance, with the number of users on it. A read-only table page: search on Enter, a total, page X / N, prev/next, one page at a time from the server.

It answers "who is on this instance, by organization" without reading the user list, and is the quickest way to spot an unexpected domain.

Related, but different: the invite-domain field in E-mail, and per-organization automatic membership by domain (Organizations / auto add users with domain name).