Microsoft working to fix Exchange Online mailbox quarantine issue

Microsoft is addressing an Exchange Online outage (EX1436407) that began on July 19, where an infrastructure change caused excessive memory use and incorrectly quarantined user mailboxes, disrupting email send/receive and calendar access. Cleanup of excess indexing data is underway and mailboxes are being restored regionally, but Microsoft has not provided a full remediation timeline or the number of affected customers; the incident is presented as an operational service fault rather than a security breach.