"Gmail and Hotmail would still mark my emails as spam whenever they felt like it. Sometimes, even for emails that were a reply to an email coming from their own servers. I almost lost a client because of that once. Maybe it was because of the IP range (I was in a cheap Scaleway server)?"

Like you, I have properly set up DKIM and SPF and blah blah.

I also have a 12 year "clean" IP from he.net.

Sometimes gmail will mark something I send as spam that is, like yours, a reply to their user and is a recipient that I have a years long history of corresponding with.

That's frustrating but what really makes me angry is that there is no indication given that this occurs. Why can't google send a proper bounce or warning message ... or even an error code that I could at least see in my logs ?

In the arms race between spammers and (providers) are error indications exploited in some way ?

Or is google just being lazy ?