The problem is due to It a number of incompatibilities (not complying to the vCalendar standard) between Microsoft Exchange style meeting invites and what iCal can understand. The safest way is to install the YAI (You Are Invited) plug-in for Apple Mail that translates and transfers Outlook, Google Calendar, Zimbra and Exchange clone meeting invitations to iCal.

With YAI installed Exchange style invites received in Mail’s inbox are automatically processed as they arrive and are sent to iCal, ready to be acted on. The incompatibilities YAI fixes include problems with Exchange style invites from different time zones, invites that generate warnings about the recipient not being on your ‘me’ card and differences in the way event updates are handled that result in error messages in iCal.

For more information about YAI and how to get a copy visit the developers site by clicking on this link.