Amavisd shows how it makes decision:
X-Spam-Status: No, score=0.476 tagged_above=-1000 required=6.2
tests=[DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1,
FREEMAIL_FROM=0.001, HTML_MESSAGE=0.001, RCVD_IN_DNSWL_LOW=-0.7,
RDNS_NONE=1.274, SPF_PASS=-0.001, TVD_SPACE_RATIO=0.001]