Are those hosts using hosted VPS instances, or are they sending through SES? There's a pretty significant difference... FWIW, I get why a lot of VPSes simply block email hosting altogether. It makes it a bit harder for me to find a host for my own small server, but I do understand the pain. Some services are better or worse, and I can imagine at the scale of many cloud hosts, trying to keep the IPs for general hosting out of blocklists would impact the bottom line more than reputational damage for a handful of legit email hosting accounts.

TBF, the demo app referenced in TFA and depending on how many emails you actually send for however many domains may well be a better option for me than my small MTA server.