Although we worked through this via PM, I'm posting what we found here for the benefit of anyone else that might be following this thread.

After reviewing the affected account, it turned out that appointments were not missing, but they were displaying on different dates in Outlook for Mac than they were in SmarterMail. Each of the mismatched appointments turned out to be all-day, annually recurring appointments. Based on our investigation, it appears that the problem occurred because the started/end dates did not match an occurrence of the recurring set. For example, one appointment had a start date of February 2, 2010, and was scheduled to recur every year on January 3. In such situations, SmarterMail treats the start date as the earliest possible date that an occurrence could happen. Outlook for Mac, however, seems to only recognize the "Yearly" portion of the recurrence rule. For the example given, this means that SmarterMail would display the appointment every January 3 starting in 2011, while Outlook for Mac appears to display it every February 2 starting in 2010.

To resolve this situation, I recommend modifying the start/end dates or recurrence rule so that they match. For the provided example, this means either changing the start date to January 3 or change the recurrence date to February 2.