Reset the local cache of an Exchange, or IMAP account

Lately I’m having some sync issues with my mailbox. For instance, I see several new emails in my mailbox when I login to webmail which I don’t see in Outlook.

Also, it looks like the entire month of June has gone missing in Outlook but the emails are luckily still available on the server.

Is there any way to get Outlook to sync or download these emails again?

Mailbox Cache buttonCorruptions in the cache are rare (especially for Exchange based accounts) but can still happen under various circumstances like an unexpected shutdown or crash of your computer or Outlook.

In these cases, it might be best to reset the local cache of Outlook and let it rebuild. The process is fairly simple to perform but make sure you do some checks first.

Step 1: Verify that the messages still exist on the server

Verify buttonThe first thing to check is whether all messages that you expect to be on the mail server (via webmail) are still there as well.

You don’t actually have to check each and every message. With sync issues you are usually missing emails in Outlook and not in webmail. However, you do want to pay a bit more attention to folders you recently moved emails into and your Sent Items folder.

If you are missing any emails on the server, either drag & drop them out of Outlook and into a folder in your Documents folder (via File Explorer) to save them as msg-file or export/copy them to a pst-file.

Both methods will allow you to restore these items to Outlook and/or the mail server later on.

Drag & drop messages from Outlook into File Explorer to create msg-files.
Drag & drop messages from Outlook into File Explorer to create msg-files.

Step 2: Backup any folders marked with: On this computer only

Backup Outlook buttonIf any of your folders of your mailbox in Outlook are marked with “This computer only”, you’ll need to copy or export the contents of these folders as well before resetting your cache.

These folders are likely to exist when you only have an IMAP account configured in Outlook 2013 or Outlook 2016.

A good method to find these folders is by switching to the Folder List Navigation in Outlook (keyboard shortcut CTRL+6). Common folders that are marked with “This computer only” are;

  • Drafts
  • Contacts
  • Calendar
  • Tasks
  • Notes

Extra Tip!
Configure Outlook to store the “This computer only” folders within a pst-file so you won’t risk losing them and also making it easier to back them up. For more details see: Don’t risk losing your Contacts and Calendar when using IMAP in Outlook 2013 or Outlook 2016

Step 3: Reset the cache of your account

OST-file buttonNow that all your unique data from the cache is backed up, you can reset the cache in the following way;

  1. Open the Account Settings dialog;
    • Outlook 2007
      Tools-> Account Settings…
    • Outlook 2010, Outlook 2013 and Outlook 2016
      File-> Account Settings-> Account Settings…
  2. Select the Data Files tab.
  3. Select the mailbox for which you want to reset the cache.
    This will most likely refer to an ost-file, but for IMAP accounts in Outlook 2007 and Outlook 2010, this will be a pst-file.
  4. Click the “Open File Location…” button.
    If your ost-file is located in the default location, then this will open the following location in File Explorer;
  5. Close Outlook and wait for it to be fully closed (just wait 2-3 minutes to be sure).
  6. Return to File Explorer and rename the ost-file of your account to .old so it will now be called like; <filename>.ost.old
  7. Start Outlook again and it will automatically rebuild the ost-file. Give it sufficient time to sync (download) your mailbox again.

Opening the file location of an ost-file via the Data Files tab in Account Settings. (click on image to enlarge)
Opening the file location of an ost-file via the Data Files tab in Account Settings. (click on image to enlarge)

Step 4: Restoring backed up items

PST-file buttonIf you backed up any data in step 2, then you restore these items after Outlook has finished syncing all your data.

If you backed them up as msg-files, then you can simply drag & drop these files back into Outlook.’

If you created a pst-file, then you can connect to this pst-file in Outlook via;

  • Outlook 2007
    File-> Open-> Outlook Data File…
  • Outlook 2010
    File-> Open-> Open Outlook Data File…
  • Outlook 2013 and Outlook 2016
    File-> Open & Export-> Open Outlook Data File

You can then drag & drop these items to their correct folders within Outlook.

After a sync interval, these items should be available again within webmail as well, unless the items are stored within “On this computer only” folders.

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