The user is not receiving any new emails on their Microsoft 365 email account.
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.
If there are no Inbox and sweep rules, it may simply be being caught by the Focused/Other inbox.
- 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.
- 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