There is a persistent belief among some senders that the technical part of sending email is the most important part of delivery. They think that by tweaking things around the edges, like changing their rate limiting and refining bounce handling, their email will magically end up in the inbox.
This is a gross misunderstanding of the reasons for bulk foldering and blocking by the ISPs. Yes, technical behaviour does count and senders will find it harder to deliver mail if they are doing something grossly wrong. In my experience, though, most technical issues are not sufficient to cause major delivery problems.
On the other hand, senders can do everything technically perfect, from rate limiting to bounce handling to handling feedback loops through authentication and offer wording and still have delivery problems. Why? Sending unwanted mail trumps technical perfection. If no one wants the email mail then there will be delivery problems.
Now, I’ve certainly dealt with clients who had some minor engagement issues and the bulk of their delivery problems were technical in nature. Fix the technical problems and make some adjustments to the email and mail gets to the inbox. But with senders who are sending unwanted email the only way to fix delivery problems is to figure out what recipients want and then send mail meeting those needs.
Persistent delivery problems cannot be fixed by tweaking technical settings.