Microsoft Outlook has some security built in to help prevent external applications from accessing its internal data. This data consists of your contacts, email addresses on emails, and others. This is meant to help prevent a virus from harvesting information to then spread itself to other potential victims. Where does leave the legit companies that provide extensions to Outlook?
There are a few different tricks that can be performed on the Microsoft Outlook client computer than can attempt to circumvent the security warnings that the user sees. These techniques have their limit, however, when the client is connected to Exchange Server.
If the client is connected to Exchange Server and they are getting security prompts from an installed add-in, the only solution is to trust the add-in from Exchange. There is a great write up about this at SlipStick. SlipStick has a lot of information regarding Outlook and Exchange. If you are experiencing the security prompts in this environment, check out the SlipStick site.