Post fix configuration / to flush / remove mailq

# postfix flush
OR
# postfix -f
To see mail queue, enter:
# mailq
To remove all mail from the queue, enter:
# postsuper -d ALL
To remove all mails in the deferred queue, enter:
# postsuper -d ALL deferred
 vi /etc/postfix/main.cf
relayhost = 172.16.7.185