Hello Mahesh,
I assume your Spam Server is up ( you can telnet 783 port directly to check if the port is up), and then what you need to do is allow external host(s) to access this Spam Server.
You can try edit
/etc/default/spamassassin
And find
OPTIONS="--create-prefs --max-children 5 --helper-home-dir"
Then add -d and -A options to allow your mail server to access this daemon.
e.q. Assumed your mail server ip is 192.168.1.5
OPTIONS="-d -i 0.0.0.0 -A 192.168.1.5 --create-prefs --max-children 5 --max-conn-per-child=128 --username spamd --helper-home-dir -s /var/log/spamassassin/spamd.log"
You can man spamd to get more opitions to set.
Hope this does help.
rds
Juan Lai