My idea of how a modern mailing service should work
From one perspective, I can totally understand why small companies want to outsource handling outgoing mail to a dedicated mail provider. The days when you could just install a MTA, plug in some settings, and be done are long over; these days doing a decent job of sending mail and getting it delivered to as many places as possible requires a significant amount of specialized expertise, and the expertise goes up if you want to use HTML mail. You could learn all of this, but why? It's better to outsource and let full-time specialists handle it for you.
On the other hand, as a sysadmin on the receiving end of these mail services I have some issues. Specifically, they get abused by spammers and they have a strong incentive to spend as little money as they can get away with on preventing this (money spent preventing spam is pure expense). On average, the only contact I have with a mailing service is being sent some form of spam (there are many mailing services and I don't sign up with very many places that use them).
Thus I have formed a theory about how such a modern mailing service should work: normally and by default it should proxy outgoing email through your server, using a dedicated proxy agent (not an MTA that you set up). All of the hard work would still be done by the mailing service on their machines and you would continue interacting with them as normal; it's just that the final delivery would emerge from your machine, on your IP address, instead of directly from one of their IP addresses.
The advantage for everyone is that this would make your mail unambiguously your mail, and avoid any contamination with other people who are also using the mailing service provider. The mailing service provider would effectively become less of a provider of mail and somewhat more a provider of mail handling software (and expertise), software that just happened to run on their servers as a service.
This clearly doesn't work for everyone in all situations, so the mailing service would still have an option to send out the mail for you. But I think that 'the mail comes out your IP address' should be the default starting case.
(Since this is the era of running companies out of AWS, it's possible that I'm drastically underestimating how many people would need the mailing service to send out email for them; maybe you simply can no longer assume that people have dedicated IP addresses in address space that hasn't been badly abused and contaminated.)