Necessary but not sufficient
With all the emphasis on getting the technical right, there seem to be people who think their mail will be delivered as long as the technical is right.
Getting the technical right is necessary for good inbox delivery, but it’s not sufficient.
The most important part of getting mail to the inbox is sending mail users want. In fact, if you’re sending mail folks want, interact with and enjoy then you can get away with sloppy technical bits. Look, major players (eBay and Intuit) have invalid SPF records, but we’re all still getting mail from them.
There are also a lot of folks who are doing everything technically perfectly, but their mail is still going to bulk. Why? Because their recipients don’t want their mail.
Permission is still the key to getting mail to the inbox. In fact, permission is more important than getting all the technical bits right. If you have permission you can play a little fast and loose with the technical stuff. If you have the technical stuff right you still need permission.