Time to decentralize our data, people!
You can run it in your network and tightly lock down the system. We can take care of all of the email deliverability setup and operations tasks, if you want.
One advantage of running licensed software on your network is that you don't pay per-message fees, so for higher volume it's really economical compared to a service like SendGrid.
To get good results on your own server, you need:
(a) To be sending email that people want to receive.
(b) To have the technology setup correctly. It's not hard; we do it all the time for our clients.
(c) It's greatly beneficial to above ~20k messages/day on your own server. Not required, but being above that point lets the ISPs gather statistical data on your complaint and open rates, which allows reputation filters to click in.