Are you still in the old email format or the new one? To tell, look at the URL in the address bar. If it says Zimbra you are in the old one, if it says Appsuite you are in the new one. If you are in the old format, from the Inbox page go to Preferences>>>>>>Email>>>>>>>Email Safe List. Turn that on, but don't put any addresses in the field for that. After that you should get nothing by way of incoming emails. If you are in the new format, from the Inbox page click on the gear icon in the upper right corner of the page, then on Settings>>>>Mail>>>>>>Advanced Settings. There you will find the same Safe List option.