I agree with your assessment of e-mail... you either rent under a big provider or you spend countless hours playing whack-a-mole with whitelist-blacklist keepers. The big providers do this too, but they're so big it's not a major slice of their operation.

a crazy amount of money on license fees

License fees pay for development, sales, support, and profit. When you go open source you can skip the sales and profit, but you have to pick up a bit of development and ALL the support, which is considerable during times of big changes, like migration to a new desktop.