You can set an expiry time for your pending messages by following the below instructions:

1. Click on SEND SMS

2. Scroll down to the 'Pending Message Expiry' and select YES from the drop down list

3. From the drop down list select in how many hours  you would like the message with the 'Pending' status to stop trying to be sent. 

4. Click on the NEXT button to proceed to the confirm your SMS campaign screen