What is the very best way to secure you server to ensure that your customers are not going to have problem accessing their information, such as email and domain hosting etc.
The best way is to have different folder for each user and don't allow user to peep into other's folder except a few public folders which may be kept visible to all.
All servers use this strategy only that is to allot different folder to each user and store all of there files in that and a user can just view his and some other public folders only.
The best way to secure your server is to gave a firewall protecting it.This prevents there from being any problems and stops customers from having any problems accessing their data.