If you change the category in Module Admin, yes, it changes it for all groups.
The modules in the category N/A (Module Admin) should never be controlled or edited by anybody other than the server owner because changes made in those sections can seriously screw up your server if you/they do not know what they are doing.
So, to answer your question, No. You can not create a single admin user to control Email settings. (Or any other server settings.) There is NO reason for anybody but the server owner to access such settings.
The modules in the category N/A (Module Admin) should never be controlled or edited by anybody other than the server owner because changes made in those sections can seriously screw up your server if you/they do not know what they are doing.
So, to answer your question, No. You can not create a single admin user to control Email settings. (Or any other server settings.) There is NO reason for anybody but the server owner to access such settings.
