You can only delete a mailing if it hasn't been approved.  Once approved, you will need to Cancel a Mailing to prevent it from sending.

How to Delete a Mailing

1. Click 

   You do not have a valid <a href="https://apps.appf.re/me/mp/cloud?hosting=cloud&tab=overview">MultiExcerpt</a> app license.&nbsp; Please sign up for a free trial or purchase a license through the <a href="https://apps.appf.re/me/mp/cloud?hosting=cloud&tab=overview">Atlassian Marketplace</a>.

    toward the top right corner of the screen
2. Click Yes in the popup window