Over time your mailbox may grow with a large number of emails and with emails that have large attachments.  This consumes space on your computer's storage and can lead to errors.  Microsoft provides a built-in feature to Outlook that will help you reduce the size of your mailbox.

Manage my mailbox size

Notes:

1. The instructions in the "Archive older items" section should not be followed.  Do not put more items into the "online archive" prior to your "Outlook to the cloud" email migration (Sept - Nov 2021) or attempt to export mail to a .PST file (blocked).  Moving items to the online archive does not decrease the amount of objects in your overall mailbox.  Therefore, doing so will not help make your migration to "Outlook in the cloud" any faster and safer.
   
   
2. The search feature in Outlook that the article above references will function differently whether Outlook is running in online or cached mode.
   
   1. You can determine whether Outlook is running in cached or online mode by looking at the status bar at the bottom of Outlook.
      
      1. Online mode
         
         
      2. Cached mode
         
         
         
   2. When Outlook is running in online mode, the search for large items will include your entire mailbox.
   3. When Outlook is running in cached mode, only the items within the cache are searched.  Since the cache is usually configured for 6 months or less, this significantly narrows the window where you might find large emails.
   4. CoreImage laptops are made to run in cached mode and cannot be changed to online mode.
   5. CoreImage desktops are in one of two states:
      
      1. For users that already logged into the desktop prior to Office 365 being installed, Outlook is running in online mode.
      2. For users that have logged in after Office 365 installed, Outlook is running in cached mode, but can be configured to run in online mode.
         
   6. You may increase the number of items in the cache by following Microsoft's instructions for turning on cache mode.  You will see a slider that determines the timeframe of cached items.  This can be extended to encompass your entire mailbox.  WARNING:  By extending the timeframe of the cache, you may consume large amounts of disk space pending on the amount of data in your mailbox.  Please do not change this value on CoreImage devices that you share with others.