Contoso Legal LLP

3 lists need a look. Mail to an empty list reaches nobody, a list without an owner (or owned only by people whose sign-in is blocked) has nobody keeping its members right, and a big list anyone outside can mail is a phishing target. Give each list an owner who still works here, and delete lists nobody uses (Exchange admin centre, Recipients > Groups > Distribution list). To stop outside mail, open the list's Settings and untick "Allow people outside of my organisation to send to this group".

7 distribution lists. Lists that accept mail from outside can be mailed by anyone on the internet; most lists only need mail from colleagues.

Each list's direct members (the first 200 of a big list). A member that is itself a list passes mail on to its own members.