/etc/postfix/main.cf で

# TLS parameters
smtpd_tls_cert_file=/etc/ssl/certs/ssl-cert-snakeoil.pem
smtpd_tls_key_file=/etc/ssl/private/ssl-cert-snakeoil.key
smtpd_use_tls=yes
smtpd_tls_session_cache_database = btree:${data_directory}/smtpd_scache

smtp_tls_session_cache_database = btree:${data_directory}/smtp_scache
smtp_use_tls = yes 
smtp_sasl_auth_enable = yes 
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_tls_security_options = noanonymous
smtp_sasl_mechanism_filter = plain
smtp_sasl_security_options =

# See /usr/share/doc/postfix/TLS_README.gz in the postfix-doc package for
# information on enabling SSL in the smtp client.

myhostname = stfuawsc.com
alias_maps = hash:/etc/aliases
alias_database = hash:/etc/aliases
myorigin = /etc/mailname
mydestination = stfuawsc.com, localhost, localhost.localdomain, localhost
relayhost = [smtp.gmail.com]:587
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128
mailbox_size_limit = 0 
recipient_delimiter = + 
inet_interfaces = localhost

みたいに

/etc/postfix/sasl_passwd に

[smtp.gmail.com]:587    address:password

書いて

postmap /etc/postfix/sasl_passwd
/etc/init.d/postfix restart