^ this

Additionally, I would probably guess correctly that almost all spam comes from rotating ASNs these days. Aka from companies that do "growth marketing" or other bullshit that isn't a valid business but just... spamming people.

A lot of the domains that fall through the cracks for single-spam-campaigns have been taken over by botnet campaigns, so the actual owners of said domains probably don't know that their website is spamming everyone else.

But the major providers are the culprit, too, here. Gmail, hotmail, microsoft o365, mailgun ... they all don't even enforce SSL from server to server, and let through "sendmail" like spam because the spammers are paying customers to them.

Source: I am maintaining antispam [1] which I am using to combat spam, phishing, and malware campaigns targeting my customer networks.

[1] https://github.com/cookiengineer/antispam