The problem is that these MTAs get hacked or people just hijack domains.
Heck, the barrier to buying a domain is so low that this is a legit way of spamming too.
This suggests that one way of making spam unprofitable would be to require any domain which sends email to put up a bond of $100 which is forfeited if any of the big four email providers decide that the domain is sending spam.
To make this acceptable to public opinion, the forfeited bonds would to go directly to popular charities, and all existing domains would be automatically grandfathered in, so there would be no extra cost for any current business or user.
Yes, but the customer will blame you when e-mail from a misconfigured MTA doesn't arrive (from my experience running one).
If only we'd had DKIM from the start, preventing thousands of broken servers from being set up in the last twenty five years...