TL;DR: modern email providers don't care if you are in blacklists. If your IP/domain does not have established reputation, they will drop half of your email in spam folder. If users whitelist it or reply to it, your reputation automatically improves.
If your send too much email or your receivers blacklist you (delete without reading or manually move your email to spam), your reputation takes nose dive. Some providers (for example, Yandex) openly describe that logic in their FAQs.
However, as someone who uses SpamAssassin (via FastMail), spam clearinghouses like Spamhaus are still very important. And as long as we want to avoid centralizing all email in the hands of a few massive providers, they will continue to be important.
And for mail senders, SpamAssassin, especially in a comparatively large deployment like FastMail’s, is super useful because it actually tells you why a message was classified as spam, and it’s mostly actionable. SpamAssassin’s rules won’t be the same as what Gmail uses, but there will be strong similarities in many of the rules, and so the sorts of actions that may be necessary to get your SpamAssassin-assigned X-Spam-score down are likely to help on providers like Gmail too.
(I just wish there was better documentation of what all the rules mean, and how to satisfy them.)
