https://www.cyberciti.biz/tips/howto-postfix-flush-mail-queue.html
Under Postfix MTA, just enter the following command to flush the mail queue:
# postfix flush
OR
# postfix -f
# mailq
OR# sendmail -bp
To remove all mail from the queue, enter:
# postsuper -d ALL
To remove all mails in the deferred queue, enter:
# postsuper -d ALL deferred