Emails transaccionales que llegan.
Una API que da gusto usar.

curl -X POST https://zend24.com/api/v1/emails \
  -H "Authorization: Bearer sk_live_xxxx" \
  -H "Content-Type: application/json" \
  -d '{
    "from": "Facturación <facturas@tudominio.com>",
    "to": "cliente@ejemplo.com",
    "subject": "Tu factura de julio",
    "html": "<h1>Gracias por tu compra</h1>"
  }'

const res = await fetch("https://zend24.com/api/v1/emails", {
  method: "POST",
  headers: {
    "Authorization": "Bearer sk_live_xxxx",
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    from: "Facturación <facturas@tudominio.com>",
    to: "cliente@ejemplo.com",
    subject: "Tu factura de julio",
    html: "<h1>Gracias por tu compra</h1>",
  }),
});
console.log(await res.json());

import requests

requests.post(
    "https://zend24.com/api/v1/emails",
    headers={"Authorization": "Bearer sk_live_xxxx"},
    json={
        "from": "Facturación <facturas@tudominio.com>",
        "to": "cliente@ejemplo.com",
        "subject": "Tu factura de julio",
        "html": "<h1>Gracias por tu compra</h1>",
    },
)

<?php
$ch = curl_init('https://zend24.com/api/v1/emails');
curl_setopt_array($ch, [
  CURLOPT_POST => true,
  CURLOPT_RETURNTRANSFER => true,
  CURLOPT_HTTPHEADER => ['Authorization: Bearer sk_live_xxxx', 'Content-Type: application/json'],
  CURLOPT_POSTFIELDS => json_encode([
    'from' => 'Facturación <facturas@tudominio.com>', 'to' => 'cliente@ejemplo.com',
    'subject' => 'Tu factura de julio', 'html' => '<h1>Gracias por tu compra</h1>',
  ]),
]);
echo curl_exec($ch);

require 'net/http'
require 'json'

uri = URI('https://zend24.com/api/v1/emails')
req = Net::HTTP::Post.new(uri, {
  'Authorization' => 'Bearer sk_live_xxxx',
  'Content-Type'  => 'application/json'
})
req.body = {
  from: 'Facturación <facturas@tudominio.com>', to: 'cliente@ejemplo.com',
  subject: 'Tu factura de julio', html: '<h1>Gracias por tu compra</h1>'
}.to_json
res = Net::HTTP.start(uri.hostname, uri.port, use_ssl: true) { |h| h.request(req) }
puts res.body

package main

import ("bytes"; "io"; "net/http")

func main() {
	body := []byte(`{"from":"Facturación <facturas@tudominio.com>","to":"cliente@ejemplo.com","subject":"Tu factura de julio","html":"<h1>Gracias por tu compra</h1>"}`)
	req, _ := http.NewRequest("POST", "https://zend24.com/api/v1/emails", bytes.NewBuffer(body))
	req.Header.Set("Authorization", "Bearer sk_live_xxxx")
	req.Header.Set("Content-Type", "application/json")
	res, _ := http.DefaultClient.Do(req)
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	println(string(out))
}