Engagement, it's not what you might think
Most delivery experts will tell you that ISPs measure recipient engagement as a part of their delivery. That’s absolutely true, but I think there’s a language difference that makes it hard for senders to understand what we mean by engagement.
ISPs, and other filtering companies, profile their user base. They know, for instance, who logs in and checks mail every day. They know who checks mail every 20 seconds. They know who gets a lot of spam. They know who hasn’t logged in for months. They know who accurately marks mail as spam and who is sloppy with the this-is-spam button. They know if certain recipients get the same mail, it’s likely to be spam.
Engagement at the ISPs is more about the recipient engaging with their email address and the mail in their mailbox then it is about the recipient engaging with specific emails.