Here’s what’s happening: Apple iOS 15 is an update that includes “Mail Privacy Protection.” Mail Privacy Protection will prevent you (the sender) from knowing if an Apple user (aka someone on your list) has opened your email or not. You simply won’t know.
That’s going to mess with the open rate and IP data for a large, unknown portion of your email list.
Normally, it’s important to re-engage subscribers (because it costs way more to get a new one). Or, part ways with them and get your list squeaky clean to keep your deliverability up (unless you want your emails to land in the spam folder...).
But after the update, you won’t really know who opened your emails or who is inactive. (Starting to see the problem?)
The update is going to drop in the fall, as early as September. It’s crunch time.
And one of the first things you can do to prepare is run a re-engagement series and clean your list now. Because, once the update comes and open rates get less reliable, it's going to get harder.