When Does a Twitter Fight Become Defamation?

My fight against self-described "unhinged British witch" Louise Mensch

Hello, everybody. Thank you again for subscribing to my Substack. First, I want to apologize for being somewhat out of touch for the past week or so. I’ve been preoccupied with what I believe was a case of defamation against me. The issue was (apparently) resolved today, and I wanted to tell you about it.