My work at Zrinity affords me the opportunity to make deliverability observations across a wide range of senders. Increasingly, over the past couple of years, I've seen that connection throttles are often directly related to engagement.
Advertising revenue is harder to come by for the major mailbox providers than it was ten years ago, and at the same time, email continues to grow in popularity. The result is that major mailbox providers have had to come up with new and interesting ways of continuing services with limited resources.
The first thing the top mailbox providers implemented was a spring cleaning. They emptied and deactivated mailboxes that weren't used for some period of time. Whether it was intentional or not, the practice lead short-term to more connection throttles being applied to senders because it affected the ratio of deliveries to attempts that has long been measured by mailbox providers.
Categorizing a message as either spam or ham has always been a challenge for mailbox providers and storing spam is extremely expensive. Resource-intensive Baysian filters have become increasingly less effective in making the correct classification as spammers implemented counter-measures, such as sending only images.
Mailbox providers eventually found a solution in a metric that email marketers had been using for a decade. Click-tracking. Most users skip over the messages they recognize as spam in their inbox and only click on those they believe are important or that they don't recognize as spam yet. A click represents engagement and clicking links within an email represents a greater degree of engagement. Eventually, connections were prioritized by ip and across a range of ips by observing user behavior and applying it either to a single ip or to a bunch by looking at the feedback loop registration.
AOL has long been regarded as one of the easiest mailbox providers to deliver too. But, yesterday ReturnPath reported that senders that didn't have the best sender reputations were being deferred with the message "421 Service Unavailable". I believe the cause of this could be the adoption of the same practice of measuring engagement or a modification in that analysis.
Knowing that engagement may affect when and if your message is delivered to your subscribers, you may want to order your subscription list to send to subscribers who've been engaged in the past, first. That will insure that you deliver to your best customers first, but will not insure you deliver to the recipients at the end of your list. You may also want to unsubscribe those who have a history of low engagement, since they may be affecting your long-term deliverability.