I set up a new mail server yesterday and used SA-Exim in conjuntion with Exim to deal with processing the mail. However I noticed from the mail headers in my emails that Spamassassin thought all my mail was coming from a trusted network (ALL_TRUSTED) – even the spam mail. Consequently, Spamassassin was assigning my email lower spam scores than it normally would and more spam was getting through the cracks.
After reading about TrustPath on the Apache wiki, I found out that having your mail server behind NAT can cause this problem. To fix it, you need to define the trusted_networks variable in /etc/spamassassin/local.cf:
trusted_networks 192.168.1.0/24 127.0.0.1