The problem seems to be that salesforce doesn't do anything about spam. Their abuse address is abuse@salesforce.com, but it is a big black hole. I contacted them on twitter and they said to forward the reports to security@salesforce.com, which I did. They claimed they would look into them, and the spam did seem to stop.
However a week ago I got a spam from exacttarget (now owned by salesforce) and I forwarded it to security@salesforce.com. The reply I got was that they had unsubscribed the recipient email address from the mailing list, along with a screenshot of said unsubscribe! I replied saying I didn't want to unsubscribe, but was reporting spam, and asked what they intended to do. I haven't gotten a reply.
And now this morning I see a spam from newswatchtv, the same spammer I reported to their security@salesforce.com in November 2020 (after them saying that reporting the spam there would get it resolved). This was from smtp12-ia4-sp1.mta.salesforce.com [13.110.74.235], the main salesforce network (not exacttarget).
So, basically they are a huge waste of time, and I don't think I have ever received any legitimate emails via salesforce and am considering blocking their entire network if I can easily figure out their ip ranges.