Redirects pass link penalties since 2013. In the past, there was this SEO trick that you could redirect a domain and get all the positive link juice, all the benefits while leaving the penalties behind. All that changed in 2013 with the Penguin 2.0. That means that all kinds of satellite domains or domains you bought for SEO and redirected in the past may have passed on penalties. Your site may have good links, but those redirected may pass a penalty. You need to run a Link Detox for each redirected domain.