I have one user on my domain running Outlook 2013 (Exchange 2010) with an additional mailbox to which he (and some others) have full access.

When viewing the mailbox with cached mode (Accounts > More Options > Advanced) enabled some emails are not visible; for no discernible reason.  They are visible to other users with access to the box and are visible in OWA or when cached mode is disabled.

There is no pattern to which emails are not visible.

This has happened thrice now.  A new profile fixes it for a while but it eventually reverts.

Office has been repaired, removed and reinstalled.

Are they marked Private or Confidential?

Seems like an issue with the local OST file, most likely due to its size. Remove any unnecessary additional mailboxes or switch to using Online mode for them, ask the user to reduce the size of his own mailbox, or create a separate profile for the shared mailbox and switch to it when needed.

None of the emails are marked private or confidential.

This issue has recurred after rebuilding the user's email profile twice before, each time it resolved the issue for a period of time and then it came back.  Unless something in one of the mailboxes is corrupting the OST file I doubt it's the cause.

Other users have identical sets of mailboxes open (aside from their primary, which is of course their own) and don't have this problem.

Online mode is working for the time being, but the user does travel and would prefer to have a cached copy.

Hi,

I suggest to repair .ost file to check this issue by the following steps:

1. Exit Outlook.
2. In Control Panel, click or double-click Mail.
3. In the Mail Setup dialog box, click E-mail Accounts.
4. Click the Data Files tab, select the Exchange account, and then click Open File Location. A file explorer window opens to the location of the data file for the Exchange account. The Account Settings and Mail Setup dialog boxes will remain open, behind the file explorer window.
5. Close the Account Settings and the Mail Setup dialog boxes, then return to the file explorer window. Important: be sure to close these two dialog boxes before you delete the file. If they aren't closed, Windows may display an error message about a conflict.
6. In the file explorer window, right-click the Exchange data file and then click Delete. The next time you start Outlook, a new .ost file is created for the account.

Best Regards.