Send email from Firebase: your functions, and Firebase's own.

import { onCall, HttpsError } from "firebase-functions/v2/https";
import { defineSecret } from "firebase-functions/params";
import { Notix } from "notix-js";

const notixApiKey = defineSecret("NOTIX_API_KEY");

export const sendWelcome = onCall(
  { secrets: [notixApiKey], region: "europe-west1" },
  async (request) => {
    if (!request.auth?.token.email) {
      throw new HttpsError("unauthenticated", "Sign in first.");
    }

    // Construct inside the handler: the secret is only readable at runtime.
    const notix = new Notix(notixApiKey.value());

    const { data, error } = await notix.emails.send(
      {
        from: "hello@acme.com",
        to: request.auth.token.email,
        subject: "Welcome to Acme",
        html: "<p>Thanks for signing up.</p>",
      },
      // One welcome per user, however many times the client calls this.
      { idempotencyKey: `welcome-${request.auth.uid}` },
    );

    if (error) {
      throw new HttpsError("internal", error.message);
    }
    return { emailId: data.emailId };
  },
);

import { onRequest } from "firebase-functions/v2/https";
import { defineSecret } from "firebase-functions/params";
import { Notix } from "notix-js";

const notixApiKey = defineSecret("NOTIX_API_KEY");

export const send = onRequest(
  { secrets: [notixApiKey], cors: false },
  async (req, res) => {
    if (req.method !== "POST") {
      res.status(405).send("POST only");
      return;
    }

    // Validate the body before it reaches the send: an open endpoint that
    // relays whatever it is given is a spam cannon.
    const { to, subject, html } = req.body ?? {};
    if (typeof to !== "string" || typeof subject !== "string" || typeof html !== "string") {
      res.status(400).json({ error: "to, subject and html are required" });
      return;
    }

    const notix = new Notix(notixApiKey.value());
    const { data, error } = await notix.emails.send({
      from: "hello@acme.com",
      to,
      subject,
      html,
    });

    if (error) {
      res.status(502).json({ error });
      return;
    }
    res.json({ id: data.emailId });
  },
);