This issue has been temporarily resolved to allow operational status to resume. We will continue to monitor throughout the day and work on a full resolution in the off peak hours.
Update 04/18/2023 16:26 PM 13 hrs
Around 10AM EST we started experiencing issues with our inbound15 cluster. During the shutdown/offlining sequence the network rejected messages with error:
554 inbound15.exchangedefender.com ESMTP not accepting messages
Unfortunately, this issue affected inbound mail flow but you should not be seeing additional errors regarding this as the cluster has been removed and the issue that has caused the error in the first place has already been addressed on the rest of the network.