Scope of owamailboxpolicy-default

Ashley 20 Reputation points
2025-08-20T03:39:28.0733333+00:00

I am planning to configure the OwaMailboxPolicyDefault to block UnknownFileAndMIMETypes. From my understanding, this setting only applies to Outlook on the Web (OWA), meaning emails with these attachments will still be delivered but blocked from being previewed/downloaded in the web client. However, the same restrictions will not apply to users accessing their mailbox via the Outlook mobile app (iOS/Android) or Outlook desktop. Can anyone confirm if this interpretation is correct?

Outlook | Web | Outlook on the web for business | Settings
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  1. Darren-Ng 1,425 Reputation points Microsoft External Staff Moderator
    2025-08-20T04:56:26.0566667+00:00

    Hi @Ashley

    Thank you for posting your question in the Microsoft Q&A forum.

    Based on your description, after researching from Microsoft sources. Your interpretation is correct. Scope of ActionForUnknownFileAndMIMETypes This setting in OwaMailboxPolicy applies only to Outlook on the Web (OWA) and the New Outlook for Windows (which uses the same web-based architecture). It controls whether unknown file types/MIME types can be opened, previewed, or downloaded in the web client.

    Emails with these attachments will still be delivered to the mailbox. The restriction only affects the ability to interact with the attachment in OWA.

    These restrictions do not apply to:

    • Classic Outlook for Windows (Win32)
    • Outlook for Mac
    • Outlook mobile apps (iOS/Android)

    Those clients do not honor OWA mailbox policies. To enforce similar restrictions across all clients, you would need to use mail flow rules, Defender for Office 365 Safe Attachments, or DLP policies.

    Here is some reference information

    Set-OwaMailboxPolicy

    Create a mailbox policy in Exchange Online for Outlook on the web and the new Outlook for Windows

    OWA Mailbox Policy attributes

    I hope information above helpful. if you have any other questions, feel free to reach out


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