Install Exim4 as Backup Mail Sever
This howto assumes that you have your DNS setup correctly to point to this server as the backup MX. It is installed on the Basic Debian Server
apt-get install clamav clamav-daemon clamav-freshclam exim4 exim4-daemon-heavy sa-exim spamassassin arj unzoo
Setup SpamAssassin and Clamav scanning as normal in /etc/exim4/update-exim4.conf.conf , be sure and setup
dc_local_domains = '' dc_relay_domains ='yourdomain.com:yourdomain.net'
Make sure dns setup is correct
Remake config and restart exim4
update-exim4.conf;/etc/init.d/exim4 restart