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. If you're only sending a small campaign, it's doubtful as to whether you'll see your campaign on this screen as it will be processed too quickly.
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 the scheduled send time if the campaign is a scheduled send.
If your campaign has already been paused, then you can click the envelope icon to resume it.
Options to pause, cancel or resume your campaign can also be selected from the More actions dropdown.