Restrict Sending emails to certain groups

Hello dear Nextcloud,

It seems anyone can send email to any group under nextcloud even if they don’t belong to any group. I test it. The whole thing is open in terms of sending emails - the To: field drops a complete list showing all groups and all users of the whole nextcloud instance :0)

So, the requirement is to restrict users of sending emails to groups they do NOT belong to, hence be able to see emails to only groups they belong to and be able to see only address books which shared with their group. Is this possible? Please let me know. Thank you all!