Messages hanging in status 'To Be Delivered' in Adapter Engine

We have a load of messages being sent out via a file adapter in FTP mode that are in this status. Since this is an asynchronous message, I am at a loss to know how that message status gets updated.

When the Adapter Engine has a message in status 'To Be Delivered', what is it actually waiting for, what is the condition by which the message status is updated from 'To Be Delivered' to 'Success'. Is there an acknowledgement signal in this case from the target systems FTP server that would be sent back to XI that would then update the message status?

What could cause a message to stay in this state?