Postfix

Do as root:

postconf -e 'relayhost = smtp.gmail.com'
postconf -e 'smtp_sasl_auth_enable = yes'
postconf -e 'smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd'
postconf -e 'smtp_sasl_security_options ='

echo "smtp.gmail.com   gmailusername:passw0rd" > /etc/postfix/sasl_passwd

postmap /etc/postfix/sasl_passwd
service postfix restart

Source: http://www.howtoforge.com/postfix_relaying_through_another_mailserver

http://wiki.dreamhost.com/Postfix#Relay_to_Dreamhost

Extra reading

• http://www.davidgrant.ca/ssmtp_vs_postfix_no_contest