Please note that this is only scoped to classic file
attachments. OneDrive and SharePoint links in the body of the emails will still
need a connection to be opened.   

For this functionality to work you need to make sure Offline
is enabled and offline attachments are enabled.   

At the user level you can verify this by checking two
settings in new Outlook Settings - General - Offline:   

1. Enable
   offline email, calendar and people  


2. Include
   file attachments  

This functionality will also be enabled or disabled based on
the True/False behavior of the already existing Mailbox Policy:
OWAMailboxPolicy- OfflineEnabledWin. 

Attachments from emails within the timeframe and folder
scope selected in the "Settings - General - Offline" will be synced
and available locally for users to open without an internet connection.  
This message is associated with Microsoft 365 Roadmap ID 472026

When this will happen:
Targeted Release (Worldwide): We will begin rolling out on mid-May 2025 and expect to complete by late May 2025.
General Availability (Worldwide): We will begin rolling out on late June 2025 and expect to complete by mid-August 2025.
General Availability (GCC): We will begin rolling out on late August 2025 and expect to complete by mid-September 2025.

How this will affect your organization:

Classic attachments from emails within the timeframe and
folder scope selected in the "Settings - General - Offline" will be
synced and available locally for users to open without an internet
connection:

What you need to do to prepare:

The default on or off of this feature will depend on your
existing Offline configuration in the new Outlook via the following policy:
OWAMailboxPolicy- OfflineEnabledWin.  This feature will
not change the default of this Policy. You can find more information about this Policy here: Set-OwaMailboxPolicy  

If you have not made any changes to this Policy, then the
default of this feature will be on.