Gmail can do their own tracking on what emails a user opens. If they find that a user is leaving a lot of mail from a domain unread, they might decide to start

Gmail can do their own tracking on what emails a user opens. If they find that a user is leaving a lot of mail from a domain unread, they might decide to start shifting that mail into the spam box to provide a better UX. As a sender, it's impossible to get any specifics from Google as to when it happens. You can get a generalized spam report rate from them but that's about it.