Hi @Alex Rourke,
Thank you for posting your question in the Microsoft Q&A forum.
According to your concern, I have tested with my New Outlook desktop app (version 1.2025.1119.200). For the result, I can still drag and drop the email or attachment in both Desktop and File Explorer. However, I must have the email clicked in the Inbox pane first and then I can able to drag and drop that email. If I choose email 1 then drag and drop a different email (email 3), Outlook doesn't allow me to do that.
You can try double checking this if you're experiencing the same behavior. If the issue still persists, I greatly suggest letting all of our users go back to using Outlook (Classic) as you intended to prevent any inconveniences or trouble this issue may have caused to your workflow.
Since this is an unexpected behavior of New Outlook and there is currently no official article or announcement addressing it, you should submit your feedback through the Outlook (new) · Community portal. This is the most effective way to ensure your voice reaches the product team and can be considered for future improvements.
Once you've submitted your feedback, feel free to share the link here in the comments. I’ll be happy to upvote it to help bring more visibility.
Additionally, since many other users have expressed similar concerns but may not know where to submit feedback, if my response has helped you better understand the situation and guided you toward a possible next step, please mark it as the “Accepted Answer.” Your action will help pin this post to the top, making it easier for others in the community who are experiencing the same issue to find your feedback and support it.
Thank you again for your time and for raising this important usability concern.
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