'By sending an email that repels all but the most gullible the scammer gets the most promising marks to self-select, and tilts the true to false positive ratio in his favor.’
[1] https://www.microsoft.com/en-us/research/wp-content/uploads/...
Ultimately if you're in the business of spamming people on the other side of the world in the hope that 0.001% of them will ultimately send a money transfer worth a month's wages in local currency, your time probably isn't so valuable you can't afford to deal with everyone that replies
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.)
It doesn't affect the average user and it presents very nominal hoops for the high volume user to step through while erecting substantial barriers to criminals.
That's kinda the best you can do