

Yeah, it is very important to consider how dependant you are on third parties. At the very least the more dependence the more power they have over you. But also how screwed you are if they just go under.
- If you use SaaS they can interrupt your use at any time and you can only react (for example demanding a reversal or lawsuits).
- If you host closed source software they can’t interrupt service on an existing contract but can legally require you to stop using it if they don’t renew the contract. (And if the company goes under you can likely get away with using the software as long as it doesn’t need code fixes.)
- If the software is open source you can continue using the software indefinitely including making code fixes. (Maintenance may be expensive as it is now your problem but that can be costed and an exit plan made if required.)
Reverse DNS is different than static IP.
But yes for outbound email, if you can’t control reverse DNS you will have pain. (Inbound is totally fine) You can in theory just use whatever hostname the ISP’s reverse DNS resolves to however you will get some spam score (or be rejected) as it doesn’t match your “from” domain.
Outbound email is a huge pain really no matter what. Unless you have a long-term lease on the IP and it isn’t in a bad network you really have to pay someone else if you want reliable delivery.