> Both of those cases are very real and happen often

"Often" but as a tiny fraction of all sender cases, something that anyone who's actually worked with email spam would know, so you either dishonestly withheld that fact or you don't know what you're talking about.

Also, low-volume spammers are quite clearly an oxymoron - the definition of "spam" includes high volume. It's literally high-volume by definition: "unsolicited usually commercial messages (such as emails, text messages, or Internet postings) sent to a large number of recipients or posted in a large number of places"[1]

And, the vast majority of individuals do not send high volumes of email. This is a fact. It is an edge case.

> because of your lack of experience

You seem to be making a lot of arguments from authority, yet you don't even have the credentials to back them up.

> You're acting like it's hard to show good behaviour for a while.

They're definitely not. It's crystal clear that the usefulness of Hashcash is not dependent on it being hard to show good behavior for a while, and in fact its most effective in the long term.

> But they also use their own inboxes to build up volume

Something that Hashcash solves, because they'll have to spend a lot of cycles solving challenges to send non-spam email to build up reputation repeatedly, because for each "clean" IP/server they'll have to repeat the "reputation ramp-up" that every legitimate sender only has to do once.

> hijack accounts

Literally no anti-spam situation protects against this, so it's irrelevant, and I don't know why you're bringing it up.

> abuse SPF policies and DKIM misconfigurations

Then these need to be fixed. Google indiscriminately blocking smaller senders isn't a fix.

> Things regular senders can do, spammers can do, but they'll spend more effort to do it at scale!

Exactly - that's what Hashcash does, is force spammers to spend more effort on the things that regular people do, because spammers do it at scale far more often than regular people do, and so it introduces a disproportionate cost on them.

> All you'd add is wasted compute resource and time. Compute and time that especially spammers get for free by abusing others.

This conclusively illustrates that you have no idea what you're talking about. Spammers do not get resources for free - they have to either spend time to acquire botnets and hijacked domains themselves, or pay money for someone to do it for them. This is pretty clear to even people who aren't in the industry.

Spamming only happens if its profitable, and the reason why its still profitable is because the cost of sending a spam email is currently low enough. Hashcash directly increases the cost of spending spam email by making it compute-bound on top of needing to acquire domains, IP addresses, and machines, and spending time establishing a good sender record, while (if combined with a reputation system) barely penalizing normal users in the long run.

Let me state it again: every new sender has to go through a reputation ramp-up process, but spammers are uniquely penalized by Hashcash because they have to repeatedly because after they start sending spam mail, they lose their reputation and have to start that process again. Normal users do not have to do this - they start a new server, solve a bunch of challenges, and then they don't have to do that again because they don't regularly burn their acquired reputation.

The fact that you can't actually counter these arguments about Hashcash sounds almost like a spammer who's worried about the deployment of an effective system based on Hashcash.

[1] https://www.merriam-webster.com/dictionary/spam