Email 'clutter' folder
Office365 email has a folder called 'Clutter'. It's possible for messages to end up here and be overlooked.
The clutter feature "moves your low priority messages out of your inbox so you can easily scan for important messages. Clutter analyses your email habits, and based on your past behaviour, it determines the messages that you're most likely to ignore. It then moves those messages to a folder called Clutter, where you can review them later" (Microsoft).
The Microsoft page about the feature is https://support.office.com/en-US/article/Use-Clutter-to-sort-low-priority-messages-in-Outlook-on-the-web-fe4d64ca-bf73-48f1-91b4-9a659e008bce#bkmk_on
To turn off the feature so that all mail is delivered to your Inbox, click on the settings cog, then select Mail. In Mail Options, select Automatic processing, then Clutter. Select Don't separate items identified as Clutter and click Save.