The easiest way to do this, would be to modify
/etc/mail/spamassassin and set the trusted_networks setting. You should
include localhost, and either your servers IP, or the entire range of
IP’s that are on your server. If your servers IP address was 1.2.3.4,
an example would be:
trusted_neteworks 127.0.0.1 1.2.3.4