Synchronization issues between Outlook and Outlook Web App

Applies to: Outlook 2013Microsoft Outlook 2010


When synchronization issues occur in Microsoft Outlook 2013 or Outlook 2010, you may experience one or more of the following symptoms.

Symptom 1

You see differences or mismatches between the messages that you receive in Outlook compared to those that you receive in Outlook Web App (OWA).

Symptom 2

You experience an issue in Outlook, but that issue does not occur when cached mode is disabled.

Symptom 3

You occasionally receive error messages when you synchronize your Offline Folder file (.ost) in Outlook with your mailbox on a server that's running Exchange Server.

Symptom 4

You notice that some items such as email messages, appointments, contacts, tasks, journal entries, and notes are missing from your .ost file or from your mailbox after you synchronize your .ost file and your mailbox.


When these symptoms occur, or when you have other problems synchronizing, the problem might be a corrupted .ost file.


If there's an issue with a specific folder, you can resynchronize the folder in Outlook 2013 or Outlook 2010. To do this, follow these steps:

  1. Right-click the folder, and then click Properties.
  2. Click Clear Offline Items, and then click OK.
  3. On the Outlook ribbon, click the Send/Receive tab.
  4. Click Update Folder.

If this method does not resolve the issue, we recommend that you rebuild the .ost file. To do this, delete the .ost file, and then let Outlook create a new .ost file and download the information again from Exchange Server. To do this, follow these steps.

How to rebuild the .ost file

  1. Locate the following folder in Windows 8, Windows 7, or Windows Vista:

  2. If the folder is not displayed, unhide the folder. To do this, follow these steps:
    • On the View menu, click Options.
    • Click the View tab, select the Show hidden files and folders check box, and then clear the Hide extensions for known file types check box. Click OK.
  3. Rename any .ost files by changing the .ost extension to .old. If the .ost file name extensions are not displayed, make sure that you clear the Hide extensions for known file types check box, as described in step 2.

    Note If you receive a “File is in use” error message, press Ctrl+Alt+Delete, and then click Task Manager. Click the Processes tab, and verify that Outlook.exe and Winword.exe are not displayed in the list of processes. If these processes appear in the list, select each file, and then click End Process.
  4. After you rename the .ost file, start Outlook. You will receive a "Preparing for first use" message. Wait for the mailbox to synchronize. The time that this process takes will vary, depending on the size of the mailbox. 
  5. After the mailbox is synchronized, test to see whether the issue still occurs. If it still occurs, the .ost file is not the cause of the issue.

Important There's an exception to this conclusion: when you have local data that's not present on the server. In that situation, we recommend that you follow these steps:
  1. Export the data to an Outlook Data File (.pst), and then delete the .ost file.
  2. Let the server data download again, and then import the .pst file data by using the Do not import duplicates option. To access this option, click File, and then click the Open & Export option in Outlook 2013 or the Open option in Outlook 2010.

Did this fix the problem?
  • Check whether the problem is fixed. If the problem is not fixed, contact support.