You need to know how long a message will remain in the Delivery queue when there are issues preventing the delivery.
Release : 10.8.0
By default, Messages will remain in the Delivery queue for five (5) days. This is configured in the SMTP Advanced Settings page which can be accessed following these steps:
Other related settings are the retry intervals, the bounce report time-out, and how long a message remains in the Delivery queue before a notification is sent. Information on these settings is available here: