My experiences are few years old, so I don’t remember excact models anymore, but some back-ups models (es series rings a bell, but as I said, it’s been a while) had batteries with soldered connectors and form-factor which (at least at the time) wasn’t available from anyone else than APC.
Also if you’re running an email server for others, it takes very little from single individual, like a small webshop newsletter, which enough people manually marks as junk and you’re on a block list again. Latest one with microsoft took several days to clear, even if all of their tools and 1st tier support claimed that my IP isn’t on a black list.
I’ve jumped all the hoops and done everything by the book, but that still doesn’t mean that any of the big players won’t just screw you up because some of their automaton happens to decide so. That’s why I’m shutting my small ISP business down, there’s no more money to make on that and a ton of customers have moved to the cloud anyways, mostly to microsoft due to their office-suite pricing. It was kind of fun while it lasted, but that ship has sailed.