Mailbox automatically Pinning every new incoming email - Desktop and Mobile

hi ev2522​ Since the issue is happening across Classic Outlook, New Outlook, and mobile, and you've already rebuilt profiles and reinstalled the clients, it sounds like the problem is following the mailbox rather than being specific to Outlook itself.

I'd check Outlook on the web first to see if the messages are also pinned there. If they are, that would point more toward a mailbox or server-side issue.

If the messages are only appearing at the top in certain clients, I'd also verify that they aren't being sorted or flagged in a way that makes them look pinned.

Given how much troubleshooting you've already done, I think the next step would be to open a Microsoft 365 support case and have the mailbox configuration checked from the service side. There may be something affecting the mailbox that isn't exposed through the normal Outlook settings.

At this point, I wouldn't keep rebuilding profiles or reinstalling Outlook, the fact that the behavior persists across multiple clients is the biggest clue.