In queue messages are messages that have been created in the system and are waiting within the Outbox section for their assigned time to be sent out.
An example of this with your Automated Reminders, they will be generated on the day a patient is booked for an appointment, but will sit in the Outbox until the assigned reminder send date.
You can reference how many recent In Queue messages you have from your the default Summary page from the In Queue column:
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