Wait what? I thought major players like GMail, Microsoft etc. have some kinda "monopoly" on eMail because they are gatekeeping by using a strict white list.
Sending emails is hard because there is no central authority. Instead, each provider checks if the sender does some “best practices”. And the exact details vary - so if you got it working once with gmail, then there is no guarantee it will work next time with live.com. So you got to do them all.
For example, I have heard an unconfirmed rumor that one of the major provider started penalizing email sender machines without IPv6 address. If I had to maintain a mail sender, this’d mean have to upgrade my architecture to use IPv6 ASAP, which is non trivial at all. This is why most people outsource email sending.
Do you know how to read email headers? Are you subscribed to mailing lists? If yes, go to your inbox and check a few messages for the sender. You would be surprised at how many senders are there.
For transactional emails / regular mailing lists, I estimate that you just need like 0.25 FTE working on email, maybe even less. While this is not something everyone can do, there are tens of thousands email senders out there.