Important: this article applies to Outlook for Windows only. 

There are two ways to connect Outlook to an Exchange account: Online Mode and Cached Exchange Mode. While Online Mode maintains a direct connection to an Exchange server, Cached Exchange Mode creates a local copy of the mailbox data stored on the Exchange server in an offline data file (.ost file). Outlook accesses this cached copy for most operations speeding response times. Outlook and the Exchange Server synchronize the local and server data periodically. This may help improve the performance of Outlook on slow Internet connections. 

To enable Cached Exchange Mode, follow the steps below for your version of Outlook:

Note: by default Cached Exchange Mode is enabled when adding an Exchange account.

For system recommendations for using Cached Exchange Mode, see the section Outlook Cached Mode Recommendations below.

In Outlook 2010/2013/2016:

  1. In Outlook, go to the File > Account Settings, then click on Account Settings again in the dropdown.
  2. In the Email tab, select your Exchange account and click on Change.
  3. In the Exchange Server Settings window, check Use Cached Exchange Mode and click on Next.
  4. You should be back at the Email Accounts window. Click on Finish.

You should see a notification window that advises you to restart Outlook in order for the change to take effect:



When you re-launch Outlook, it will begin to make a copy of your Mailbox content to your computer. This will take some time. Not all items will be immediately available. If you have a large mailbox or a slow Internet connection, it can take an hour or more. 

In Outlook 2007:

 

  1. In Outlook, go to Tools > Account Settings.
  2. In the Email tab, select Microsoft Exchange Server and click on Change.
  3. In the Exchange Server Settings window, check Use Cached Exchange Mode and click on Next.
  4. You should be back at the Email Accounts window. Click on Finish.

You should see a notification window that advises you to restart Outlook in order for the change to take effect.



When you re-launch Outlook, it will begin to make a copy of your Mailbox contents onto your local computer. This will take some time. Not all items will be immediately available. If you have a large mailbox or a slow Internet connection, this can take an hour or more.

Note: if you want to turn off Cached Exchange Mode, follow the same steps, but make sure that the box next to Use Cached Exchange Mode is unchecked.

Outlook Cached Mode Recommendations:

  •  It is not recommended to use Cached Mode when the mailbox size is greater than 25 GB as it might cause performance issues.
  •  By default, the overall size of .ost files has a preconfigured limit of 50 GB.

Outlook 2011/2016 for Mac:

Note: unlike Outlook for Windows, Outlook for Mac 2011/2016 always runs in Cached Mode and it is not possible to switch it to Online Mode.