No, it can be a different way and still not have spam, by trusting/tracking domains instead of IP addresses.
If you filter by IP block (or address!), it might be a block that has changed hands and is no longer spammy. Or it might be a block from the Zen Policy Blocklist, which blocks ranges that the responsible ISP has submitted as domestic or retail blocks that are supposed to send outbound mail through the provider's smarthost.
If you filter by domain, that could be the envelope sender, the From:, the Reply-to:, or the domain of the SMTP client. Only the last is reliable; and you also have the IP address for the client. In my experience, the IP address is more useful, for longer, than the domain name. But any good blocklist should age quickly (i.e. old stuff should drop off the list).
Depends on your approach. If you hack IoT devices then you have a lot of IP's. If you hack Joomla sites, you have a bunch of domains.
> I don't know - is free domain tasting still a thing?
Yes. There are also discounts and stuff like that.
In your case, it's likely that your volume and sending patterns aren't consistent and trustworthy enough to keep track of your domain and IP reputation.
You have to understand that they get millions of letters from new domains each day, sent from compromised Wordpress blogs and the alike. If you want to be deliverable, you have to be consistent and not suspicious.
Or, more likely, there's some other mistake in your configuration somewhere.
The need to warm up new IP's has existed for a while and a lot of providers do it. Any postmaster with experience knows how and why it's done.
Alternatively - come up with something akin to D&B registration system so people can attest that they won't engage in spammy behavior.
Yes, and I'm saying what's the prequisite for that to happen. As long as it's okay (which it currently is) to send unsigned mail, IP addresses have larger weight. DKIM needs more deployment for that to change.
There's absolutely no way that IP-based reputation schemes will be deprecated before alternatives are viable. Sure it would be nice for a few people here, but no, won't happen before the ecosystem improves.
> Alternatively - come up with something akin to D&B registration system so people can attest that they won't engage in spammy behavior.
Already exists. That too gets abused.