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
Create a mailbox policy in Exchange Online for Outlook on the web and the new Outlook for Windows
I hope information above helpful. if you have any other questions, feel free to reach out
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