Use an OWA Mailbox Policy to Block Attachment Download for the New Outlook for Windows

The ConditionalAccessPolicy setting in an OWA mailbox policy can be configured to work with Entra ID conditional access so that OWA blocks access to attachments on unmanaged devices. Microsoft originally introduced the feature in 2018 and as it turns out, the combination of OWA mailbox policy and CA policy also blocks attachment access for the new Outlook for Windows client.

https://office365itpros.com/2025/05/13/owa-mailbox-policy-new-outlook/