All filters are not equal
Many questions about delivery problems often assume that there is one standard email filter and the rules are the same across all of them. Unfortunately, this isn’t really the case.
The biggest divide is consumer versus business filters. Business filters don’t really care about things like engagement. A sender could have near perfect engagement with a message to a business. But a decision maker inside the company can still decide that mail doesn’t get in. There’s no appealing to permission or wanted mail. Employee mail is provided for the good of the business, not for the good of the individual user or the sender.
There are other less obvious divides between filters as well.
I frequently refer to “webmail providers” (Oath, Microsoft, Gmail). These are companies that control the mail delivery and, for the bulk of their customers, control the mail client as well. They can use engagement filters because they have more data. Other companies, like broadband providers or web hosting services, don’t have the same level of access to customer behaviour, so they can’t heavily use engagement as part of their filtering processes. They may have some access to IMAP folders, depending on their setup, so they can look at some engagement flags.
Filtering companies also have their own type of filters. In many cases, though, they have no access to any engagement filters. They handle mail at a discrete point that starts during SMTP sessions and ends when the mail is handed off to the local delivery agent. These companies cannot use engagement as part of their filtering process all, they simply don’t have access to that data.
Understanding what data filters act on and what data they have access to can inform how to deal with blocks and delivery problems.