Assume that you run the Microsoft Outlook 2013, Outlook 2016, Outlook 2019 or Outlook for Office 365 client in cached mode. When you close your Outlook client and then reopen it later, changes that were made to your mailbox items while Outlook was closed aren't synchronized as expected.
This issue most frequently occurs when mailbox items are changed by using Exchange Web Services (EWS) calls. These changes are sometimes performed by archiving solutions.
Under these conditions, the synchronization failure is expected for Outlook in cached mode.
To work around this issue, use one of the following methods:
- For cached Outlook clients, clear offline items from the folder where the sync failures occur. To clear offline items, right-click the folder in question, select Properties, select the General tab, and then select Clear Offline Items.
- Use Outlook in online mode.
Outlook depends on several MAPI properties to govern cached mode synchronization. The specific properties that Outlook uses are PR_CHANGE_KEY and PR_PREDECESSOR_CHANGE_LIST. In this case, the values for these two properties are decremented instead of incremented when messages are altered through EWS calls, and that action leads to the sync failure.