To empty the que, run this command:
postsuper -d ALL
Before you empty the queue, you can try to find out which script has send the mails by inpecting the mail content with the command:
postcat -q /path/to/the/mailspol/file
To find the path of the mailfile, you may run:
and then search the file with:
where [MAILID] is the ID of a spool item in the postqueue -p listing.