Note: if you are NOT using the *Debian* package, then you don't have this file and you don't have the init script referenced below. Check with your operating system vendor to determine the proper course of action.
Edit /etc/default/spamassassin to start `spamd' at boot time and to and not add a From_ header (a line starting with ``From '') at the top of the messages. The From_ header will really break your mail because the From_ header is only for mbox mailboxes and is used to separate messages.
# Change to one to enable spamd ENABLED=1 OPTIONS="-F 0"
Now start spamd (as root) :
# /etc/init.d/spamassassin start