Microsoft is working on addressing an Office 365 issue that has resulted in legitimate emails sent from multiple domains (including Google and LinkedIn) getting tagged as malicious. Microsoft says the issue stems from an accidental block placed on valid and healthy domains, which resulted in legitimated emails inadvertently being quarantined. Microsoft has fixed the bug that led to legitimate emails getting quarantine and is now unblocking users still unable to send new messages. Microsoft Defender for Office 365 and Microsoft 365 Defender users should expect to see additional impact including an increase in the number of URL-related alerts for non-malicious URLs.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-office-365-is-blocking-emails-from-google-linkedin-domains/