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Email messages, calendar, tasks and other items are saved on a server and your computer, on your computer, they're saved as Outlook Data Files (.pst or .ost). Just like any file, there is a possibility that the Outlook Data Files can become corrupted. This can be seen as missing emails, emails displayed with the wrong info in them or Outlook not being able to run at all.

This fix is for Microsoft Exchange Email accounts only, which means a .ost file.

This fix assumes that you have access to Outlook but could just as easily be performed by going directly to the folder where your .ost is stored.

Rename and recreate a corrupted .OST file
Typically you can fix syncing errors by either renaming or recreating your Outlook Data Files. 
Here are the instructions for Outlook 2010 and 2013

Locate the .ost file.

   1.  In Outlook click the File menu, then click Info
   2.  Click Account Settings, then click Account Settings again
   3.  Click the Data Files tab
   4.  Click the data file that is set as the default, or the one which you need to change
   5.  Click Open Folder. This is an easy way to get to the location of your .ost
   5a. If you can't get into Outlook just do a search for *.ost
   6.  Close Outlook

Rename the .ost file by right clicking on the file and selecting Rename.

   - We suggest rename the file to ost.old

Re-open Outlook. Outlook should start to configure a new data file, but will retain all previous settings, including signatures and autocomplete email addresses.

   - This process can take awhile depending on your mailbox size and network speed.

When your mailbox has been rebuilt you'll see the message, "All folders are up to date" in the lower right hand corner.