How SpamAssassin defines BAYES_99 ? How to avoid it?

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?

What is BAYES_99?

BAYES_99 is a statistical component of SpamAssasin. It is using some sort/variation of Naive Bayes which has (to keep it simple) some sort of probability saying that some word/token is likely to be spam or not to be spam.

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.