Restrict sending emails to specific groups

The autocompleting feature for sending to nextcloud groups https://github.com/nextcloud/mail/pull/810 is absolutely great!
But to avoid unwanted spamming there should be some restriction options.

In the Settings -> Sharing section the option "Restrict users to only share with users in their groups" restricts sharing to own groups only. In our case we want to enable the users to share files with other groups (so disable this option), but they should not be able to mail to users in other groups.
Even if this option is ticked we need more restriction. E.g. we use the "Everyone Group" https://github.com/icewind1991/group_everyone because we need some global file sharing for all users. Sending mails to this group (= all users) should be permitted completely or allowed to admins and/or group admins only. (There's an request to restrict all sharing for this specific group to admins only https://github.com/icewind1991/group_everyone/issues/11 but in our case we do need the file sharing, only disable mails and chat.)

Background: We are a school with teachers, students, parents, office and so on (will be about 1.000 users). We have content that should be available for everyone like downloadable files, school calendars... But in no way everyone should be able to send mails to all 1.000 users. As groups we have classes, teachers with subgroups, working groups. For some working groups it makes sense to get mails from anybody, but for most groups, classes and teachers not. File sharing between groups is needed.

My suggestion would be an option in the Settings -> Sharing section "Restrict sending emails to groups (exclude groups)" and/or "Restrict sending emails to groups (include groups)", working like the "Exclude groups from sharing" option where groups can be selected.

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Start in Settings → Sharing and review the existing group-sharing restriction described in the issue, then trace how group recipients are handled for mail. Confirm the intended include/exclude behavior and admin or group-admin exceptions before implementation. Done means mail restrictions can differ from file-sharing permissions without allowing unintended group-wide mail.