Command to view, force send and delete queues
| View Queued messages | mailq |
| Delete all Queued messages | postsuper -d ALL |
| Force sending mail | sendmail -q |
| View Message content in Queue | postcat -q <Queue Id> |
Reference: http://www.cyberciti.biz/tips/force-sendmail-to-deliver-a-message-in-sendmails-mail-queue.html