The below is what I do to delete an email from a Postfix queue.
First we need to locate the email that you want to delete from the postfix queue
This will list the emails in the active queue
The first number is the message ID, so to delete the email with ID 99ADC288CF from the postfix email queue we would type the following;
postsuper -d 99ADC288CF
This will delete the email with the ID 99ADC288CF from the postfix email queue.
The above assumes that you have console or ssh access and that you are logged in as root.
You must exercise extreme caution while deleting emails from the queue.
We will not be held responsible if you delete the wrong email from the postfix queue or cause any other form of damage while following this guide, you do so at your own risk.