Sending, receiving, and organizing email in Outlook.com
Hi,
May I ask if you are currently using the Mail app or the Outlook app?
Does this happen after an app update?
Does this only happen on this specific account?
In the meantime, try to remove or log out of your account in the app, then restart your device. Log in again. Observe for a few hours to see if the emails will sync.
Please let me know how it goes. I look forward to your response.
Stay Safe!