Gmail tracks email reputation. It’s part of what gives Gmail some of the best spam filtering on the planet. Gmail looks at how much email your domain sends to @gmail.com addresses and assigns a score to your domain based on how engaged those @gmail.com addresses are with your content. When your domain crosses a threshold, you get labelled as suspect. The Gmail scoring system used to apply only to your reputation within Gmail. Google is now applying email behavior to search results. If you don’t keep your email list clean, you get slapped with a penalty.