The company I work for was sending confirmation emails doing everything properly: separate IP range mx boxes, warmed up, DMARC, SPF, DKIM, all the bells and whistles. Nothing else was allowed to go through them.
Suddenly people weren't getting those confirmation letters. It turned out that soon after Gmail introduced the "Promotions" tab, a silent, new feature was added: anything from noreply@ was put in there.
We tried going through the official channels - once we got to the end of the tunnel, it told us "we'll get back to you in 2 weeks". That's when we started digging into internal google connections across the company, and thank god, we found someone who know someone, and in was sorted within a couple of hours. (It might have helped that the company spends a lot of money on google ads.)
Gmail is lovely.