Just as Parmi said, it is not feasible to stop the sent items and junk emails synchronization with Office 365 Web App ( OWA).If you concerned about the feature with OWA, it is recommended that please give a feedback to
https://outlook.uservoice.com/forums/313228--outlook-on-the-web-office-365, and post your good ideas there. Microsoft is always focusing on customers feedback and using experience,
some new features would be added to the office 365 products and services based on customers feedback in the future.And your good ideas will be very helpful for them to improve the products and services, thanks.
Your understanding will be highly appreciated.
@Parmi Thanks for your sharing.
* Beware of scammers posting fake support numbers here.
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