How SpamAssassin defines BAYES_99 ? How to avoid it?

Question

SpamAssassin marks my email with reason BAYES_99 , which means the email is 99% ~ 100% spam email. What is the mechanism of SpamAssassin to detect this rule (by keyword?), and how can I avoid hitting this rule?

Answer 1

I found this because I was using SendGrid for transactional mail, and sendgrid was sending MY mail from a server that had been blackholed for sending SOMEONE ELSE'S spam. Because the IP was blackholed, my emails were marked spam. To diagnose:

You can see if your domain or IP has been blacklisted here: https://mxtoolbox.com/blacklists.aspx

The fix was to switch to a different email transport provider.