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

To see mail queue, enter:
# 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