Microsoft is rolling out a new Office 365 feature dubbed ‘Unverified Sender’ The feature is designed to help users identify potential spam or phishing emails that reach their Outlook client’s inbox. The new indicators will be shown in the user’s Outlook inbox for messages where the client couldn’t verify the sender’s identity using email authentication techniques. Microsoft is also rolling out increased DKIM key sizes to increase security in all environments during October. The feature might not be available yet for all users as a result of rolling out now.
Source: https://www.bleepingcomputer.com/news/microsoft/office-365-now-warns-about-suspicious-emails-with-unverified-senders/