If you attempt to archive a Campaign before it completes delivering messages to all of the contacts, you receive a helpful warning message letting you know that not all contacts have been sent.
You can confirm that you would like to continue with archiving the Campaign which will result in permanent failure of all remaining undelivered messages; or,
You can abort the archiving request and allow the Campaign to continue to run through all queued messages
Please note that stopping/archiving a campaign due to an error with the Campaign will not stop most messages from being sent. Please be sure to double-check your Campaign for accuracy before clicking launch!

If a Campaign is archived before it finishes delivering its queue of messages, undelivered messages in the queue may or may not be delivered. Additional detail will display in the Analytics dashboard.
Messages still in the queue at the time of archiving may still be delivered! There is a point, quickly after a Campaign is launched, where once the messages that are queued to send, stopping/archiving the Campaign will not pull the messages out of the queue.
Messages that are able to be pulled back when a Campaign is stopped/archived before all messages have finished sending will be permanently reported as “pending” in analytics (not as failed).
A warning message will show on the Campaign’s analytics dashboard that pending messages are a result of a Campaign being archived too early
If a launched Campaign has some contacts who are in a "Still in Queue" status (meaning some contacts in the Campaign have not yet been delivered, but the Campaign is still trying to send those messages), it is reasonable to wait 24 hours and see if those initial undelivered contacts can retry to send. After 24 hours, it is recommended to manually archive the Campaign, as the undelivered contacts will likely try to resend the Campaign message indefinitely.