https://www.bleepingcomputer.com/news/microsoft/office-365-now-warns-about-suspicious-emails-with-unverified-senders/

Microsoft is currently rolling out a new Office 365 feature dubbed 'Unverified Sender' and designed to help users identify potential spam or phishing emails that reach their Outlook client's inbox.

"Unverified sender is a new Office 365 feature that helps end-users identify suspicious messages in their inbox," says the company on the new feature's Microsoft 365 roadmap entry.