Exchange Mailbox Too Large? Check The “Top Of Information Store”

Recently, CQL had a problem with some Exchange mailboxes being too large even after deleting emails. One of the users had an Exchange mailbox capacity of approximately 2.7GB. However, looking at the folder size (in Outlook) showed around 1.2GB actually being used. Here’s an example of what we saw:

We pulled up the folder size using the Exchange Management Shell with the following command:

  • Get-MailboxStatistics username | Select-Object DisplayName,ItemCount,TotalItemSize
  • username is the username of the mailbox

You can easily pull up size data on everyone on the server by using this command:

  • Get-MailboxStatistics –Server servername | Select-Object Displayname,ItemCount,TotalItemSize | Sort-Object TotalItemSize –Descending
  • servername is the servername of the exchange server.

We wanted to examine why we there was a difference in the totals (where were the extra messages coming from). I looked at the folder size using the following command:
  • Get-MailboxFolderStatistics username | Select-Object Name,ItemsInFolder,FolderSize | Sort-Object FolderSize –Descending | Select-Object –First 10
  • username is the username of the mailbox


This showed us the largest folder was called “Top of Information Store.” There are two main ways to solve this problem: Disable the Outlook Today Home page (didn’t show all the messages) or use mfcmapi.exe. The latter worked!

Download mfcmapi.exe and get MFCMAPI 64 bit executable if you have 64bit Outlook. Now do the following:

  1. Open mfcmapi.exe
  2. Click Session
  3. Click Logon
  4. Select your Outlook Profile
  5. If you are doing this for a different mailbox than the one that is local on the machine do steps 6-8, if you are doing it for the local mailbox just double click on the user in the main window.
  6. Click MDB > Open Other Mailboxes > From GAL…
  7. Select the User, click OK
  8. Click OK
  9. Expand the Username
  10. Right click on Top of Information Store
  11. Click Open contents table
  12. Let the window will most likely be blank for 10-30 seconds, just wait.
  13. The window will then start populating with messages, on the bottom right you can see how many are loaded, wait until all of the messages are loaded.
  14. Then select all the messages (CTRL + A) and press the delete key
  15. You now have the option to send them to the users’ deleted item, Perm delete them to the deleted items retention folder, or a hard delete.
  16. Select what you want and click OK.
  17. Now just wait, the program will more than likely be in a not responding state. This is fine, do not keep clicking on the window or kill it with task manager, just wait. Depending on the amount of messages this will take a while. Eventually, it will start responding again and the messages will be gone.
  18. That’s it, after doing that the mailbox was reduced in size.

Why Did This Happen?

These messages are in this hidden folder because of Rules. Check all of the users' Rules setup in Outlook. When there are Rules setup to move messages to a certain folder, sometimes the user deletes the folder and not the Rule. This then defaults the Rule to send the message to the Top of Information Store, which is the very top folder you can choose (your e-mail address). Fix all the Rules and no more messages should go to that folder.

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