Microsoft Engineers have identified an issue which caused a higher than expected volume of email within the transport service. As a result some internal email messages were delayed by at least 15 minutes. An update has been implemented which has mitigated further mails from being delayed and any remaining delayed messages are being processed. Microsoft engineers will continue to monitor until all message queues have drained and mailflow has been returned to normal. The next update will be provided on September 23rd, at 23:45 PM UTC.