Here are some postfix queue management commands that I find myself using regularly (on an Ubuntu server):
mailq: Check the current postfix queue.postcat -q 8523E2FA336F: View message stuck in queue with ID8523E2FA336F. The ID can be found in the mailq output.postqueue -f: Flush the queue; Force postfix to attempt sending the mail immediately instead of waiting for a while.postsuper -d 8523E2FA336F: Delete message in queue with ID8523E2FA336F. The ID can be found in the mailq output.