I just looked through a variety of some of the most recent spam messages I’ve received, with Fastmail’s SpamAssassin setup, with a user Bayes filter (I’ve easily had enough spam to achieve that), Vade checking, and various block lists (some pertaining to sender, some other properties).
It’s a surprisingly even mixture of content, sender-related metadata, other message-related metadata, and unattributable (e.g. SH_HBL_EMAILS, ME_VADESCAM). Most of the time, any two of those four would be enough to reach the spam threshold of 5. Regularly, any one of at least three of them.
I should note that what I’m calling “sender-related metadata” is not penalising unknowns: it’s only penalising known-bads. Thus, it’s not really about sender reputation as a whole, but rather established bad sender reputation. The only form of penalising of unknowns that I’m aware of with Fastmail is when the sender is on a domain name registered in the last I think 72 hours.
When it comes to the more tailored things (oh, you somehow managed to spend two hours looking at my site, particularly liking my Rust FizzBuzz article, and wonder if I wouldn’t mind sharing a link to your Python guide, and you keep pestering me?), it’s only content, with everything else neutral. (In the specific example I cited there, the first message got BAYES_00, the second got BAYES_50, and the third BAYES_99 + BAYES_999 perhaps due to me manually marking the previous ones as spam but probably also from introducing the term “guest post” which I imagine my Bayes filter regards dimly.)
(I like the fact that I can inspect Fastmail’s spam filtering to quite some degree, and you can talk to their support about it as well and get more detail when desired. The big ones like Gmail are just completely opaque, with people poking and prodding at the edges to try to understand its caprice. Disclosure: I worked for Fastmail for a few years.)