Milly brings up a VERY good point below regarding the possibility
of a BES (Blackberry Server) being a point of failure.

Also you may want to check the settings in your Outlook for the following:

Select the TOOLS menu from Outlook
Select OPTIONS
Select E-mail OPTIONS
Select the TRACKING OPTIONS

Look for "After processing, move receipts to...
  Delete blank voting and meeting responses after processing

Turn this off.

OR

Select the TOOLS menu from Outlook
Select OPTIONS
Select CALENDAR OPTIONS (button)
Select RESOURCE SCHEDULING... (button)

Then look for:

Use these options if you are responsible for coordinating resources, such as
conference rooms.
  Automatically accept meeting requests and process cancellations.

Turn this off.

Nikki Peterson