Thank you for posting your query!
It seems that while email encryption via Azure Information Protection (AIP) works fine in the classic Outlook desktop client, it may not yet be fully supported in the "New Outlook" or Outlook Web App (OWA). I understand how important it is to have consistent functionality across different platforms.
As of now, the "New Outlook" and Outlook Web App do not support email encryption features that are available in the classic Outlook desktop client. This is a known limitation, and many users have expressed similar concerns. Microsoft is actively working on improving the new Outlook experience, so it's possible that these features may be added in future updates.
In the meantime, if you need to send encrypted emails, I recommend continuing to use the classic Outlook client until the new versions catch up in functionality. You can also keep an eye on Microsoft's official updates this feature.
Appreciate if you could share the feedback on our feedback channel. Which would be open for the user community to upvote & comment on. This allows our product teams to effectively prioritize your request against our existing feature backlog and gives insight into the potential impact of implementing the suggested feature.
For more details, please refer: Security and compliance in new Outlook for Windows
I hope this information helps. Please do let us know if you have any further queries.
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Thank you.