Unable To Open A Large Exchange 2016 Mailbox

Aftab Alam | Updated On - 07 Apr 2021 |

Read time 6 min

The Exchange mailbox of a user is the repository of all the professional communication. As the number of mailboxes increases in the database and the volume of data increases in mailboxes, the size of the Exchange database file (EDB) grows. There are many examples when the users are unable to open a specific mailbox which has a large size, and its items are inaccessible to the user. Also, Microsoft has set a maximum size limit for databases (the limit varies according to the version and the edition of Exchange Server). For example, in Exchange 2013 the size limit of a database is 16 TB, but the Exchange Experts recommend going for a maximum size of 2 TB only. Although 2 TB is quite a large size itself and if the network bandwidth is not sufficient enough, then it may be troublesome to access the mailbox data. In such a scenario, you need to use different methods to deal with the mailbox size as well as the database size.

First, check the size of the mailboxes using the following cmdlet.

Get-MailboxDatabase -Status | select ServerName,Name,DatabaseSize

To reduce the size of large mailboxes, you need to delete unwanted messages from the mailbox. You can search for messages to find and delete unwanted messages.

Here is the cmdlet for the same:

Search-Mailbox -Identity “Mailbox name” -SearchQuery “Search criteria'” -DeleteContent


Search-Mailbox -Identity “Test Mailbox” -SearchQuery “Subject: ‘Test Message'” -DeleteContent

Also, you can try methods like archiving, attachment extraction, etc.

To deal with the size of the database, you have two options which you can choose. First, either defrag the database file or change the registry settings to suit the current size of the database. Let’s go through both processes one by one –

Change registry settings

Follow the steps:

  1. Open Regedit and go to the path-
  2. Check if it shows the database size limit in gigabyte under the ‘Private- [database GUID],’ then modify the size limit. If it does not show, then create a DWORD ‘database size limit in GB’ and input value.
  3. Cancel the Registry Editor and restart the Exchange Information Store.
Defrag the Exchange Database

You should use the ESEUTIL tool to defrag the database which will remove the white space from the database and reduce its size.

  1. Check the white space in the database in Exchange Management Shell (EMS)
    Get-MailboxDatabase Database3233 -Status | Format-List Name, DatabaseSize, AvailableNewMailboxSpace
  2. Dismount the database (in EMS).
    Dismount-Database Database3233
  3. Run the defragment cmdlet in ESEUTIL.
    Eseutil /d C:\DB\Database3233.edb
  4. Mount the database back to Exchange Server (in EMS).
    Mount-Database Database3233

Now you can notice a considerable improvement in the performance of Exchange Server.

A professional tool to deal with large mailboxes

If mailboxes are still not opening in the Outlook, then you will have to go for a professional tool which can export the large mailboxes into multiple PST files.

Kernel for Exchange Server recovery is a unique software which can remove all the issues related to a database file and save the recovered data at multiple destinations. When you are facing a problem with a large mailbox, you can export it into multiple PST files. Later, you can access these PST files in Outlook. Here is the process of using the software:

  1. Launch the software and click the Offline EDB icon.
    Open software and click on Offline EDB
    NOTE: You can also click the Add Source option for the same job.
  2. Select the first option of Offline EDB File, then click the Next button.
    Select Offline EDB File and click the Next button,
  3. Select the EDB file from its location; otherwise, search it from any drive or folder. Click the Next button.
    Select the EDB file from its location
  4. There are two scan modes which you can choose – Standard Scan and Deep Scan. Choose any one and click the Next button.
    Select Scanning Mode
  5. The tool recovers the EDB file items. Click the Finish button.
    The tool recovers the EDB file
  6. Now select the large mailbox which was not opening. Right-click it and choose the option Export Mailboxes to PST.
    Export Mailboxes to PST
  7. Here, you can check/uncheck to select/deselect various folders. Apply filters as per need and most importantly, check the ‘Split PST file after’ option. It will break the PST file after it reaches a specified size. Thus, you can get multiple PST files for a large mailbox. Browse the destination where you want to create the PST files and click the Export button.
    Split PST file
  8. After successfully exporting the PST files, the tool gives you a message. Click OK.
    Export PST Completed
  9. The software also provides a report detailing the status of PST files and location of the source mailboxes and destination PST files. Click OK.
    Generate report of PST files

Whenever you face a situation where the size of the mailbox has become much large and it is not opening, then use Kernel for Exchange Server and divide the mailbox into multiple PST files. The benefit of these PST files is that you can open them in Outlook instantly. The data will be saved in the same hierarchy as in the Exchange mailbox.