If you're running (or recently upgraded to) Microsoft Office 2008 for Mac connected to an Exchange server then there's a good chance you will encounter one or both of these warning messages every time you launch Entourage and authenticate to the Exchange server:
Despite Microsoft's acknowledgment of the issue since mid 2008 (two years ago!) there is still no quick and easy fix, and no way to suppress this message. The good news is the message is simply annoying and won't stop you from checking your email.
Our research, testing and experience has been that until Microsoft releases Office 2010 for Mac the only solution (outside of clicking OK on a daily basis) is to properly install an Exchange Server generated root certificate on each Mac workstation experiencing these messages. More information is below: