1)To list all mail of queue, use one of the following commands.
postqueue -p or mailq
2)To delete or flush all emails from Postfix mail queue using the following command.
postsuper -d ALL
3) You can only delete all deferred emails only from mail queue. Use the following command to delete deferred emails from the queue.
postsuper -d ALL deferred
4) If you want to remove any specific email. Use the following command to remove specific emails only. First search the ID of that email like below command
postqueue -p | grep “email@example.com”
ex: 056CB129FF0* 5513 Sun Feb 26 02:26:27 email@example.com
5) Now delete the mail from mail queue with id 056CB129FF0.
postsuper -d 056CB129FF0