I didn't imply HTTP API, I said "API" on purpose: they provide an SMTP API for bulk sending (an smtp relay), simple smtp sending and a nice curl command that can be used in simple scripts.

If that is not enough, Nylas is an option.

I just find extreme limiting your hosting options for email sending, especially when you can still host wherever you want and use email services from other providers.

In case of self hosting, the clients are also limited (usually yourself, family and friends), so you can even ask to "allow list" the address you plan to use for those communications.