If Permission is King, then Engagement is the Emperor, my esteemed coworker Karen Balle explained to me this morning. And she's right.
Engagement (or lack of engagement) seems to be causing a lot of bulk foldering issues lately. If your mail is going to the bulk folder at a top ISP, it's probably going to be because recipients don't care about your email. They're not engaged.
What is engagement? I could break it down, but why re-invent the wheel? George Bilbrey provided a nicely detailed breakdown of engagement metrics, how they work, and what ISPs do with them.
To me, this highlights yet again that an email address isn't forever. Keep mailing somebody forever, or send them irrelevant messaging, and they're going to get bored with you. (Even if they had opted in!) Your open and click rates drop off significantly, and ISPs pick up on that. They will denote that most of the subscribers don't care, that recipients never read the email you send. And that means that the ISP isn't going to feel compelled to ensure that mail goes to the inbox. If the recipients don't care, the ISP isn't going to care either.
George points out that engagement isn't a new thing, but I would counter that engagement is now a much bigger deal than it was, say, two years ago. Or go back even further, to the days where a single spam complaint would get you blocked. You'd protect against it with double opt-in, and the ISP would keep the gates open for you. Now, permission isn't enough; you have to make sure what you're sending is wanted by recipients.
Of course, reputation still matters significantly, as do all the things that have historically gone into reputation metrics, most importantly, permission. Chip House and I will touch on reputation and engagement, along with a panel of experts, next week at our Connections '09 Conference.