as this behave is not expected. And only meeting request be effected. So I suspend it as an Stale rule issue. it could use mfcmapi to edit.
Download MFCMAPI from http://mfcmapi.codeplex.com/releases/view/66930
1.) Run MFCMAPI and hit ‘OK’ to clear the ‘About MFCMAPI’ information dialog.
2.) Select Session from the dropdown menu and click “Logon and DisplayStore Table”.
3.) Choose the Outlook Profile needed if set to prompt otherwise if there is only one profile it will load that one by default.
4.) Double-click the Default Store for the user’s mailbox which is tagged as True
5.) Expand the ‘Root – Mailbox’ item. Highlight Inbox
6.) Select Actions from the dropdown menu and click “Display rules table”.
7.) Check the Stale Rule by compare the rule shows in outlook and MFCMAPI.
8.) Select the Schedule + EMS Interface (Delegate Rule) and delete it.
this state will be at column PR_RULE_PROVIDER which will only exist in a Mailbox on the Server and NOT in a PST file
Remember, if there are multiple delegates assigned this will remove them all and the currently expected Delegate must be re-configured.
9.) Close windows to the main MFCMapi Screen and select Session dropdown menu and hit Logoff.
More information http://technet.microsoft.com/en-us/library/bb508857(EXCHG.65).aspx