Appointments accepted tentatively without user's knowledge

I have a user who is saying meetings appear on his calendar without him accepting them. He never sees the invitation in his inbox either. I have verified this by sending him a test invitation. He is a Mac user running Outlook 2011, to my knowledge this client cannot perform auto-accept, but just to make sure I had him close Outlook and launch OWA, then sent a second test invitation. This one was also auto accepted.

I checked to make sure there were no server side rules attached to his account (and no client side in Outlook). Below is the output of get-CalendarProcessing for his account. I don't think there is an issue with any of these settings. They are identical to other users who are not having this issue.

[PS] C:\Windows\system32>Get-CalendarProcessing *username*|fl

RunspaceId                          : b6f805f6-41ef-4375-aa05-0323fe7daa24
AutomateProcessing                  : AutoUpdate
AllowConflicts                      : False
BookingWindowInDays                 : 180
MaximumDurationInMinutes            : 1440
AllowRecurringMeetings              : True
EnforceSchedulingHorizon            : True
ScheduleOnlyDuringWorkHours         : False
ConflictPercentageAllowed           : 0
MaximumConflictInstances            : 0
ForwardRequestsToDelegates          : True
DeleteAttachments                   : True
DeleteComments                      : True
RemovePrivateProperty               : True
DeleteSubject                       : True
AddOrganizerToSubject               : True
DeleteNonCalendarItems              : True
TentativePendingApproval            : True
EnableResponseDetails               : True
OrganizerInfo                       : True
ResourceDelegates                   : {}
RequestOutOfPolicy                  : {}
AllRequestOutOfPolicy               : False
BookInPolicy                        : {}
AllBookInPolicy                     : True
RequestInPolicy                     : {}
AllRequestInPolicy                  : False
AddAdditionalResponse               : False
AdditionalResponse                  :
RemoveOldMeetingMessages            : True
AddNewRequestsTentatively           : True
ProcessExternalMeetingMessages      : False
RemoveForwardedMeetingNotifications : False
MailboxOwnerId                      : *********
Identity                            : *********
IsValid                             : True
ObjectState                         : Changed

What else can I check? What am I missing? Any assistance would be appreciated.