Some organizations prefer all Outlook clients to connect to their Exchange servers using Cached Exchange Mode (also known as offline mode) because clients in Online Mode can generate significantly more disk I/O traffic. Besides using Group Policy to control this from the client side, in Exchange 2007, 2010, 2013 and 2016 we can force all clients to use Cached Mode at the server end by running the following cmdlet:
Get-Mailbox | Set-CASMailbox -MAPIBlockOutlookNonCachedMode $True
The Set-CASMailbox cmdlet is used to configure client access settings on a mailbox. For example, we can configure settings for ActiveSync, Outlook, Outlook on the web, POP3, and IMAP4. The MAPIBlockOutlookNonCachedMode parameters, which is available only in on-premises Exchange servers, controls access to the mailbox by using Outlook in online or offline mode. Valid values for this parameter are:
True: only Outlook clients that are configured to use Cached mode are allowed to access the mailbox;
False: the state of the Cached mode setting is not checked before Outlook clients are allowed to access the mailbox (both online and cached mode are allowed). This is the default value.
However, be careful with this setting! If there are Outlook clients currently configured in Online mode and you set this parameter to True, they will receive the following error message and will not be able to connect until they change their Outlook profile to Cached mode: