$wgSysopEmailBans should be deprecated and removed:
- Its name is misleading; it must be true for any user to be able to ban a user from using their email
- Finer control is available via $user->isAllowed( 'blockemail' ) - for all user groups to be prohibited from banning emails, just set $wgGroupPermissions['sysop']['blockemail'] = false; - as of now, sysop is the only group given this rights in DefaultSettings.php