In reporting you will see a status option. This shows the sending status as either Preparation, Sending, Resending, Stopped or Finished.

Preparation (yellow) means it is preparing to send your message.
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​Sending (green) means it is currently sending.
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​Resending (light blue) means it has finished the initial send try and will continue to resend until it the optimal delivery has been made.
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​Stopped (red) means it has been stopped by you using the Stop Delivery option under Reports.
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​Finished (gray) means the message has been sent and is no longer retrying to send. Reports will continue to update as subscribers show activity on the delivered messages.