When synchronization issues occur in Microsoft Outlook or in Microsoft Outlook Web App (formerly Outlook Web Access), you may experience the following symptoms.
You see differences or mismatches between the messages that you receive in Microsoft Outlook compared to Microsoft Outlook Web App.
You have an issue with Outlook, but the issue does not occur when cached mode is disabled.
Occasionally you receive an error message when you synchronize your Offline Folder file (.ost) in Outlook with your mailbox on a server that is running Microsoft Exchange Server. The error message may resembles the following:
Cannot start Microsoft Outlook. Cannot open the Outlook window. The set of folders cannot be opened. The file c:\Users\<username>\AppData\Local\Microsoft\Outlook\<firstname.lastname@example.org> is not an Outlook data file (ost).
You notice that some items such as email messages, appointments, contacts, tasks, journal entries, notes, posted items, and documents are missing from your .ost file or from your mailbox after you synchronize your .ost file and your mailbox.
When these symptoms occur, or you have other problems synchronizing, the problem might be a corrupted .ost file.
Outlook 2003 and Outlook 2007
Use the OST Integrity Check Tool (Scanost.exe) to check your .ost file for inconsistencies.
Outlook 2010 and later versions
The OST Integrity Check Tool (scanost.exe) is not included in Outlook 2010 and later versions. This tool was recommended in earlier versions to fix errors in Outlook data files (.ost). If there is an issue with a specific folder, you can resync the folder in Outlook. To do this, follow these steps:
Right-click the folder, and then click Properties.
Click Clear Offline Items, and then click OK.
On the Outlook 2010 ribbon, click the Send/Receive tab.
Click Update Folder.
If these methods do not resolve the issue, we recommend that you rebuild the .ost file. To do this, delete the .ost file, and then let Outlook download the information again from Exchange Server. For more information, see the "More information" section.
However, the exception is if you have local data that is not present on the server. In that case, we recommend that you follow these steps:
Export the data to an Outlook Data File (.pst), and then delete the .ost file.
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 the File tab, and then click the Open option.
The OST Integrity Check Tool runs only on .ost files and can be used to diagnose and repair synchronization issues. It scans your local copy of the .ost file and your mailbox on an Exchange Server. The tool compares items and folders in the file and in your mailbox and tries to reconcile synchronization differences between the files and the mailbox. The OST Integrity Check Tool does not change your mailbox on the Exchange Server. The tool records any differences in a scan log so that you can see what differences the tool found and resolved. The scan log also identifies any situations that the tool could not resolve and that you must fix manually. The scan log is in your Deleted Items folder.
The OST Integrity Check Tool (Scanost.exe) is installed when you install Outlook in the following locations, as appropriate for the Outlook version that you are running.
For Outlook 2003: drive:\Program Files\Common Files\System\MSMAPI\LocaleID folder. Be aware that in this example, LocaleID is the locale identifier (LCID) for the installation of Microsoft Office. The LCID for English - United States is 1033.
For Outlook 2007: drive:\Program Files\Microsoft Office\OFFICE12
How to repair errors by using Scanost.exe
Exit Outlook 2003 or Outlook 2007 if it is running.
If you have set up Outlook to prompt you for a profile, the tool also prompts you for a profile. In the Profile Name list, click the profile that contains the .ost file that you want to check. If you are prompted to Connect or to Work Offline, click Connect.
Select the options that you want. To have the tool automatically resolve differences that it finds during the scan, click to select the Repair Errors check box. If this check box is cleared, the tool logs the problems. However, the tool does not make the necessary corrections.
Click Begin Scan.
Note To view the scan log, start Outlook, and then open the Deleted Items folder. The tool does not scan the Deleted Items folder. Any problems are noted in a message that has "OST Integrity Check" as its Subject.
If you have problems when you try to open your .ost file, you can use the Inbox Repair tool (Scanpst.exe) to diagnose and repair errors in your .ost file. The Inbox Repair tool can be used on both .ost files and on Personal Folders (.pst) files. The Inbox Repair tool scans the .ost or .pst file and makes sure that the file structure is intact. The tool also tries to repair the internal data structures, if it is necessary. The tool does not interact with your mailbox on the Exchange Server.
How to rebuild the .ost file
Locate the following folder, as appropriate for the operating system that you are running:
Windows XP: C:\Documents and Settings\<alias>\Local Settings\Application Data\Microsoft\Outlook
Windows Vista and later versions: C:\Users\<alias>\AppData\Local\Microsoft\Outlook
If the folder is not displayed, unhide the folder. To do this, follow these steps:
Windows 8 and Windows 10
On the View tab, click to select the Hidden items check box, and the File name extensions check box.
On the Organize menu, click Folder and search options.
Click the View tab, and then click to select the Show hidden files, folders, and drives option, and then click to clear the Hide extensions for known file types check box.
On the Tools menu, click Folder Options.
Click the View tab, and then click to select the Show hidden files and folders check box, and then click to clear the Hide extensions for known file types check box.
By default, Outlook creates an Outlook.ost for the user's local cache. If an Outlook.ost file already exists, Outlook inserts a number at the end of the file name (for example, Outlook0.ost or Outlook1.ost). Rename any .ost files. To do this, rename the .ost file name extension to .old. If the .ost file name extensions are not displayed, make sure that you click to clear the Hide extensions for known file types check box as described in steps 2A and 2B.
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, click each file, and then click End Process.
After you rename the .ost file, restart Outlook. You receive a "Preparing for first use" message. Wait for the mailbox to synchronize. This process varies, depending on the user's mailbox size.
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.
For a similar error caused by a different issue, refer to https://support.microsoft.com/en-us/help/3068991.