The outbox is where sent campaigns sit whilst they're being sent, or if they have been scheduled for sending.
Once all the sends are processed, the campaign moves to the Sent section.
Pausing and cancelling campaigns
While a campaign is being processed, you can click on the envelope icon under the Status column to pause or cancel your campaign. Pausing your campaign will also give you the chance to modify any scheduled send time if the campaign is a scheduled send. If your campaign has already been paused, then you you can click the envelope icon to resume it. Pausing, cancelling or resuming your campaign can also be selected from the More actions dropdown.