Problem

The user is not receiving any new emails on their Microsoft 365 email account.

Solution

This is usually the result of an Inbox rule, especially if the account was recently compromised by a phishing email. To correct this, you will need to turn off inbox and sweep rules.

To turn off Inbox and sweep rules in Office365:
  1. Log into your Office365 account using a web browser and go to Outlook
  2. Click on the gear icon in the top right corner and select View All Outlook Settings at the bottom of the side bar.
  3. From the Mail tab select Rules 
  4. If there are any Inbox or sweep rules, move the slide selector to the left to turn off the rule, or select them and click the trash can icon to delete them permanently.

If there are no Inbox and sweep rules, it may simply be being caught by the Focused/Other inbox.

  1. If you are using the Focused inbox, your mail may be in the Other folder. Click Focused or Other to toggle between the two mailboxes.
    1. To turn off Focused inbox, click Settings, then click Mail in the Your app settings section. Select Mail>Layout>Focused Inbox and choose Don't sort messages. Click Save.

See More info here : Managing the Focused Inbox with Microsoft 365