One of the most common questions we get from an author is a very simple one:

Why is this subscriber marked as undeliverable?

We're working on a whole suite of observability and functionality to tackle this (which is fancy corporate speak for we're trying to make it better). Here's one small way: in the subscriber detail view, we now will automatically highlight if the domain of a subscriber email address might be a typo or if their IP address looks to be coming from a VPN, which we have flagged for malicious activity. Often these things are not because the person signing up for your newsletter is evil; we just err on the side of false positives in order to keep your deliverability as high as possible. If you see this while browsing, feel free to change that subscriber's email address or mark them as OK.