From LedHed's Wiki
Deleting Individual Messages
To delete an individual message from the mail queue follow these steps:
Step 1) Display the contents of the queue
mailq
Step 2) Identify the message and "Queue ID"
The list should look something like this
-Queue ID- --Size-- ----Arrival Time---- -Sender/Recipient-------
C017B1A942 2328 Tue Feb 27 03:54:01 root
root
BCD0B1A93A 333 Tue Feb 27 05:22:02 root
root
259D91A955 272 Thu Mar 8 02:17:11 root
root
8ED5119EB1 333 Wed Mar 7 09:36:49 root
root
Step 3) Delete the message
postsuper -d <queue_id>
Deleting All Messages
To delete all messages in the mail queue run this command:
postsuper -d ALL