It seems to be an issue on Outlook's end. Especially if you have 2 3rd party clients trying to read from your outlook account (like Thunderbird on your comp and other one on a mobile device).
Per Microsoft help site:
It seems to be an issue on Outlook's end. Especially if you have 2 3rd party clients trying to read from your outlook account (like Thunderbird on your comp and other one on a mobile device).
Per Microsoft help site: