Currently if you allow an admin to manage users they can also send mail to all users. I believe this has nothing to do with managing users. I allow them to manage users for editing signatures, avatars or things like that if needed. I have 90.000+ registered members and I am not comfortable with giving this permission to my admins. My ip would be blocked by providers and my server would be suspended probably if such thing happens. I think there must be an additional permission called Email Users They also have the permission of rebuilding caches which would not be so nice for a large site.