What is the problem with the mail command ? Or with some msmtp ?

Except that with "standard" mail you only need a provider or self-hosted server, and not a RESEND_KEY

To me, it looks like "reinventing the wheel"... Did I miss something ?

BTW: content generation (AI, email) is not email sending, so you can still generate content with other tools and send it with a simple "mail" command