source: i worked at fastmail and regularly got oncall alerts to go deal with some rbl issues
The OP describes carefully monitoring reputation and is their own self-hosted mailing provider (not a service-provider) so presumably isn't hosting spam...
The difference may be the human contacts? Or just that self-hosted mailing providers are so rare that it's hard to keep their patterns from looking like spam to the filters? Or other?
His complaint seems to be mostly about rate limiting an IP that doesn't normally send in bulk volume. It's a weird behaviour. I would recommend he not use fastmail or gmail, but an actual provider for bulk delivery.
"backing off" people submitting mail, mail resending when failed, these are all really good useful parts of the email system that help you recover when things go wrong, even though it feels like a slap in the face - it's much better than the alternative of whitelist only, or blocking people permanently.
It was frustrating for me sometimes.... free.fr was often on our rbl list, and if I recall it was mostly because a customer (different ones at different times) would send an email to bob@free.fr an that person was actually like bobby@free.fr, so free.fr thought you were sending crap, or spam or whatever. Usually it was just an undeliverable mail.
Another thing is - IPs yes - they have "warm up" as you start sending mail out of it. In fact, providers like fastmail have a bunch of IPs, that way if one was flagged, and they figured out what was going wrong (you wouldnt just flip to the new IP, because a outbound spammer would muck up your new IP instantly), you flip over to the other IP which wasn't on any RBL.
anyway, I'm probably botching the explaination, one time our anti-spam guy drew a giant whiteboard of how it all works and it gave me an instant hangover.
Just to say - yes it's more complex than 'open up the port and start sending mail' but certainly not approaching 'no independent providers are allowed'
From your further description, it definitely sounds like something hard to do without significant dedicated staff -- like I could believe it is not realistic to just "run your own mail server" anymore, when mail isn't meant to be your business. For better or worse, you need to use someone where mail is their dedicated business, with the scale to devote significant resources to keeping on top of it. (an "anti-spam guy"!) Which didn't used to be the case in olden days. But, as you say, that doesn't mean it needs to be one of the big three.
Is my conclusion at this point.
But a "bit" (lot) more is required than one interpretation of "just taking spam seriously and having prudent rules in place" might suggest!
