Postfix

For relaying

To Enable relay for domains
Form shell

 echo "relay_domains = /etc/postfix/relay" >> /etc/postfix/main.cf
 echo "[DOMAIN NAME HERE]" >> /etc/postfix/relay
 echo "[DOMAIN NAME HERE] smtp:[IP ADDRESS TO FORWARD TO] >> /etc/postfix/transport
 postmap /etc/postfix/transport
 postfix reload

to add additional domains:

 echo "[DOMAIN NAME HERE]" >> /etc/postfix/relay
 echo "[DOMAIN NAME HERE] smtp:[IP ADDRESS TO FORWARD TO] >> /etc/postfix/transport
 postmap /etc/postfix/transport
 postfix reload

http://www.howtoforge.com/amavisd_postfix_debian_ubuntu

http://www.freespamfilter.org/FC4.html#_Toc110999175

http://flurdy.com/docs/postfix/

http://wiki.linuxquestions.org/wiki/Using_MailScanner/Postfix/SpamAssassin/ClamAV_in_Gentoo_Linux#What_is_Postfix.3F

http://www.howtoforge.com/virtual_users_postfix_courier_mailscanner_clamav_centos

http://wiki.linuxquestions.org/wiki/Using_MailScanner/Postfix/SpamAssassin/ClamAV_in_Gentoo_Linux#What_is_Postfix.3F