Can my employer sue me for defamation if I share a specific story of their mistreatment?

Hello. I now have a new job, and ready to move on. However, I'm still feeling a bit vindictive because I faced racism, gaslighting, reduction in pay, and all sorts of harassment from this asshole. How much information can I post here without getting in a legal trouble? Can he sue me if I post a screenshot of his email to me? He's in DC, by the way. ​ Thank you!