SMTP is a must. My advice is to never bother with proprietary mailer APIs - you will need to change providers sooner or later (sometimes on short notice, if your current provider temporarily suspended you on a false positive for example), which is much easier when you just need to swap the SMTP credentials vs implementing yet another proprietary API. Plus it makes local testing easier - there's no shortage of "fake SMTP for development" projects out there.

Of course, tech bros don't want you to do it, as it reduces their vendor lock-in.