#1 setup a password maps file
vi /etc/postfix/sasl_passwd
mailserver.com username:password
#2 fix owner and permissions
chown root:root /etc/postfix/sasl_passwd; chmod 600 /etc/postfix_sasl_passwd
postmap /etc/postfix/sasl_passwd
#3 edit /etc/postfix/main.cf
relayhost = mail.ispserver.com
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options =
#4 postfix reload
and you are done!
