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.
