In Office 365 there are several throttling limits that administrators may encounter that impact mail flow. One of these throttling limits is Organization Queue Quota Exceeded.
When organizations send a large amount of mail through the service – either by relay or direct submission – and the service is unable to relay (mail is deferred for some reason, transient failure, 4yz smtp response code, connectivity issues, etc) the mails are queued for delivery and retried until expiry in 48hrs. For example maybe a large number of emails are addressed to a domain with a valid MX record but no one is answering at that name the messages will remain in queue. As queues start to reach certain thresholds across our transport environment – we will begin to defer newly submitted mail with Queue Quota exceeded (Sender Queue quota if mail in our queues are from a specific sender, or Organization if there are multiple senders)
This throttling will continue to occur until the queues begin to drain. The drain of queues may occur either through delivery of the messages to their intended targets or as the messages expire in the service. In some cases it may be necessary to consult with product support services to help identify this condition and provide remediation.
The throttling limits are in place to ensure that mail queued for one tenant do not start impacting on mail deliveries process for all other tenants on the shared server infrastructure.
The way to resolve during such an incident is to identify the bad mail (typically storm /large amount of auto-generated mail) which is getting stuck in queues in our service. Then resolve the reason for it being queued – address delivery issue or stop auto-generated mail from the source if it was sent to invalid recipients. The service will auto-recover and start allowing new mail to be submitted. In some cases it maybe necessary to consult with product support services on the resolution.