According to the 2015 Email Data Quality Trends Report by Experian, 73% of marketers reported having issues with email deliverability.
Not doing so, as you would expect, results in deliverability problems.
However, the Global Deliverability Director at Oracle, Kevin Senne, reported that they had examples of compliant Certification customers having deliverability issues with some of the advertised internet service provider ISP partners.
They do report that Certification customers perform better at Gmail than non-Certified senders, something they attribute to the best practice standards they require customers to follow.
So, the big question for Certification customers is how much of their deliverability success stems from being a responsible sender versus the technical lift being provided at ISP partners.
As for inbox placement, the problem is often that panel data will show less than 100% inbox placement at Outlook and Gmail due to filtering by individual users.