MC1259820 - Offline attachments policy for the new Outlook for Windows

Introduction

We are introducing a new OWAMailboxPolicy setting that allows administrators to control whether file attachments are available when users work offline in the new Outlook for Windows. This change adds a tenant-level configuration option for managing offline attachment access while preserving existing offline policy behavior by default.

When this will happen:

The new policy will rollout mid-April 2026

How this affects your organization:

Who is affected:

• Exchange Online administrators
• Users of the new Outlook for Windows who use Offline mode

What will happen:

• A new OWAMailboxPolicy setting named AttachmentsOfflineEnabledWin becomes available.
• The policy is enabled by default ($true).
• Administrators can allow or block access to file attachments while working offline in the new Outlook for Windows.
• This change does not affect the existing configuration of OfflineEnabledWin.
• If OfflineEnabledWin is set to $false, offline file attachments remain disabled even if AttachmentsOfflineEnabledWin is set to $true.

OfflineEnabledWin	AttachmentsOfflineEnabledWin	Offline mail available?	Offline attachments available?	Description
true	true	Yes	Yes	Offline mode is enabled. Users can work offline and access file attachments.
true	false	Yes	No	Offline mode is enabled, but file attachments are not accessible while offline.
false	true	No	No	Offline mode is disabled. File attachments are also not available while offline.
false	false	No	No	Offline mode is fully disabled. No offline mail or attachments are available.

What you can do to prepare:

• No action is required if the default behavior meets your organization’s requirements.
• Review your existing OfflineEnabledWin policy configuration.
• Configure AttachmentsOfflineEnabledWin at the tenant level if you want to control offline attachment access.
• Communicate any changes to helpdesk or security teams.

Learn more and configure the policy using Exchange PowerShell: Set-OwaMailboxPolicy documentation.

Compliance considerations:

No compliance considerations identified, review as appropriate for your organization.