Gmail doesn't provide the raw FBL (Feedback loop) data. It's only available in Google Postmaster which has an API for domains you have already setup and have to

Gmail doesn't provide the raw FBL (Feedback loop) data. It's only available in Google Postmaster which has an API for domains you have already setup and have to web scrape to set new domains. Validity took over FBL for a lot of providers and just recently announced you will only get statistics or pay a yearly fee to get the raw FBL data. I also handle support for an email marketing platform. Users 100% will mark an email as spam then ask that company why they stopped receiving emails. Some will even use the unsubscribe link, write that they receive too many emails, then email asking why they stopped receiving email. It's an imperfect system.