Comment by easygenes

I was an original Thunderbird pre-1.0 (from 2003) user and prior to that, Netscape Mail, and am quite certain it has had bayesian spam filtering all this time, at least since the late ‘90s. That was a headline feature in the early days. My first email account used POP3 through a shared web host for my own domain in that era.

Edit: Yes it’s still there https://support.mozilla.org/en-US/kb/thunderbird-and-junk-sp...