Today I sent an email to a person with an attachment that was not supposed to be sent, so I had to log into my postfix server and delete it from the queue before it leaves the server.
Fortunately, I realized before it leaves, and thanks God the email was big enough for me to have time to log in, find the message and delete it.
If you ever need to delete an email from Postfix queue, you have to follow this steps.
Once logged in your server find the message you are looking for.
sudo postqueue -p
This is going to list the current active queue in your server, something like this.
5642B4D8647* 1683500 Tue Jun 3 08:37:27 firstname.lastname@example.org email@example.com 9359B4D82B1* 1635730 Tue Jun 3 08:36:53 firstname.lastname@example.org email@example.com
The first number is the message ID, the asterisk behind it, denotes that the messages is currently in the process of being sent, to delete it enter.
postsuper -d 9359B4D82B1
This is going to delete message ID=9359B4D82B1, from the queue.