Edit /etc/postfix/main.cf
smtpd_recipient_restrictions = permit_mynetworks, permit_sasl_authenticated, reject_unauth_destination smtpd_sender_restrictions = reject_unknown_sender_domain
Shows a list of files in queue
postqueue -p
postfix reload
postsuper -d ALL
to delete all the items in queue
tail /var/log/mail.log -n 100