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I knew how to uncheck the system admin for the registered people. but I need to let the registered people to use the mail user features in system admin. Therefore, once I checked the mail user option, then the system admin will appear in the page.
I try to disable the whole system admin, and re-create a link to link to the mail user option, notifications and inbox. but the problem is when there is a new message, the inbox would not display the number of new message.