Office 365 and Exchange Online send daily notifications to users when inbound email has been quarantined as a phishing email, or as spam/unwanted commercial email "UCE."
The message is sent from quarantine@messaging.microsoft.com and has the subject line "Spam Notification: __ New Messages".
The message reads:
Review These Messages
10 messages are being held for you to review as of 11/13/2020 12:00:00 AM (UTC).
Review them within 30 days of the received date by going to the Quarantine page in the Security & Compliance Center.
For Prevented phish messages, you have the options to block the sender, review the message safely.
For Prevented spam messages, you have the options to block the sender, release the message to your inbox, or review it safely.
Note that because the standard notification sender from SharePoint, no-reply@sharepointonline.com is frequently spoofed for sending malicioius email, this address is filtered and you will frequently need to manually release these messages to your inbox.