What is SpamAssassin?
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.
How is your SpamAssassin Score calculated?
SpamAssassin developers keep the factors involved in the score calculation public.
These are some key elements checked by SpamAssassin:
Main Domain and IP blacklists/greylists/blocklists
Collaborative spam identification databases
SPF and DKIM protocols
Header and Body content
Broken or shortened urls
Multiple sender reputation systems
However, SpamAssassin is a complex and dynamic program, including scores from other spam filters in its own score calculation.