What's really silly is that Intuit purchased Mailchimp at the end of 2021.  Mailchimp knows a thing or two about sending emails :).  And with Mailchimp you can also send using their Transactional Email (formerly Mandrill) service: https://mailchimp.com/features/transactional-email/. 

Now 18 months later there is still not ability to send using a custom domain.  Custom domains are clearly deliberately not supported.