Apache SpamAssassin is a filtering program that uses a variety of spam-detection techniques and provides a spam score to every email: 5 being considered as spam.
SpamAssassin is used by most ISPs (Internet Service Providers) and individual domains as a spam-filter. Meaning that a bad grade (4 or above) will impact severely your deliverability.
SpamAssassin developers keep the factors involved in the score calculation public.
These are some key elements checked by SpamAssassin:
However, SpamAssassin is a complex and dynamic program, including scores from other spam filters in its own score calculation.