If you see the below popup, then a message that you sent over 24 hours ago has not been picked up by any recipients. This is meant to notify you that you may want to investigate further.
For instance, you may want to know which files were not yet picked up by the recipients.
To check this, click on the Outbox Icon on your Home Screen. Any message you see with an Exclamation Point in the bottom left has been waiting to be picked up for longer than 24 hours.
Tip: You may want to send a reminder to the intended recipient to go back and pick up the files.