so my instinct for a while has been that Substack have been damaging their sender reputation by offering unlimited free sending to unlimited recipients, with the upshot that they attract a lot of people sending low-open-rate, marked-as-spam or marked-as-promotional emails. (They also, to my knowledge, won't let senders use their own email addresses, all emails are sent from Substack).

I realise it's much more complicated than this, that within a given ESP there are multiple bundles of senders and that better customers get moved to a better bundle etc, but my guess is that for any given level within Substack you're being bundled with an unusually-bad group of peers and therefore getting lower open rates -- I would love to know if this is an accurate understanding, or if there's any accurate way to test this from outside Substack.