If you are using Microsoft Office Outlook 2007, Microsoft Outlook 2010, or Microsoft Outlook 2013 Preview to connect Microsoft Exchange Online and you notice that some messages are missing from the mailbox, the issue may occur if one or more of the following conditions are true:

  • The message was deleted
  • The message was archived
  • Outlook is set up to use a custom view that may be filtering the missing message
  • The missing items weren't synchronized to your Outlook profile

If you are running Outlook 2013 Preview in Cached Exchange Mode and you suspect that the missing items weren't synchronized to your Outlook profile, you can check how much is synchronized to your *.ost file

  1. Click File > Account Settings > Account Settings.
    Account Settings in the Backstage view
    x
  2. Select an Exchange account, and then click Change.
    Change command in the Account Settings dialog box
    x
  3. Under Offline Settings, set the slider to All, 12 months, 6 months, 3 months, or 1 month. Cached Exchange Mode, which downloads a copy of your messages, must be turned on.
    Offline mail storage amount slider

 

See this article for more