When messages can't be sent from your Office 365 organization to your on-premises or partner email servers using connectors, the messages are queued in Office 365. Common examples that cause this condition are:
The connector is incorrectly configured.
There have been networking or firewall changes in your on-premises environment.
Office 365 will continue to retry to delivery for 48 hours. After 48 hours, the messages will expire and will be returned to the senders in non-delivery reports (also known as a NDRs or bounce messages).
If the queued email volume exceeds the pre-defined threshold (the default value is 2000 messages), the alerts will be available in the mail flow dashboard at Recent alerts, and admins will receive an email notification (to their alternative email address).
Queue alert details
When you click the alert, the alert details appear in a flyout pane.
You can click View queue in the alert details to see the queue details, problems, and links to the available fixes in a new flyout pane.
Even if the queued message volume hasn't exceeded the threshold, you can still use the Queues area of the mail flow dashboard to see messages that have been queued for more than one hour. You can use the Queues area to monitor the number of queued messages (the value 0 indicates mail flow is OK) and take action before the number of queued messages becomes too large.
When you click the number of queued messages in Queues, the queue details and guidance for how to fix the issue will appear in a flyout pane (the same flyout that appears after you click View queue in the details of a queue alert).