No matter how great an email program deliverability is, no one can guarantee that 100% of the email sent will reach the recipient’s inbox. Why? Recipients can make decisions about where mail goes in their own inbox. Every mail client has a way for users to control where mail is delivered.
This is good for delivery, when the mail means so much to people that they override spam filters and put mail in their inbox. This is problematic for delivery when the final recipient throws mail away or filters it into spam.
Of course, there’s no way to know what individual recipients are doing with mail in general. Sure, there’s currently panel data but that is only for the subset of users that installs a 3rd party app into their mailbox. There’s no way to know where 100% of email is delivered.
For me, I consider any email program with a >95% inbox delivery rate to be an excellent program. I also don’t think there’s much the sender can do in order to get that last 5% to the inbox. That 5% is just not reachable. Not by improving data, not by double opt-in, not by any of the things we do to improve delivery. Some small percentage of mail is just never going to get in front of the user.
The primary reason for these delivery failure is the end user. End users can, and do, create their own filters. While ISPs do curate the inbox, end users have the ability to filter email. If an enduser sets up a filter for a particular email, the ISP isn’t going to overrule that. ISPs want the end users to have a pleasant inbox experience. Thus, the end user’s wants and needs rule. Nothing is clearer to the ISP whether a particular user wants an email than that user directly setting up a filter.