Send transactional email with Resend and Functions

Build a Hono endpoint on Azion Functions that calls the Resend API, store the API key as an environment variable, and deploy it with Azion CLI.

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In this tutorial, you will build an HTTP endpoint on Azion Functions that sends a transactional email through Resend. You will create the project from a template, write the handler, store the API key, deploy the project, and send a live message.

The finished project is in the edge-functions-examples repository.

Prerequisites

1. Create the project from the Hono template

To authorize the CLI and create the project:

Authenticate Azion CLI

Run the login command:

azion login

With no credential flags, the CLI opens a browser-based flow. It stores the credentials locally, and every later command is authorized against your account.

Initialize the project

Run the init command in the directory that holds your projects:

azion init

The command opens an interactive journey.

Name the project

Enter a name, or press enter to accept the suggestion:

? Your applications name: (black-thor)
Select the Hono preset

The CLI lists the presets it supports:

Shell
01? Choose a preset: [Use arrows to move, type to filter]
02 Angular
03 Astro
04 Docusaurus
05 Eleventy
06 Emscripten
07 Gatsby
08 Hexo
09> Hono
10 Hugo
11 Javascript
12 ...
Select the Hono Boilerplate template
Decline the local server and the deployment

The CLI offers a local development server and a deployment. Answer N to both, because the handler is not written yet:

Shell
01Do you want to start a local development server? (y/N)
02Do you want to deploy your project? (y/N)
Install the project dependencies

Go to the project directory and install its dependencies:

Shell
01cd <your-project-name>
02npm install

The project directory holds the Hono template and its dependencies.

2. Write the handler that calls the Resend API

The handler answers a GET request on the root path, posts the message to the Resend API, and returns the API response as JSON. The entry field of azion.config.js names the entry file of the project. Open that file and replace its contents with:

TypeScript
01import { Hono } from "hono";
02
03const app = new Hono();
04
05const RESEND_API_KEY = Azion.env.get("RESEND_API_KEY")!;
06const EMAIL_TO = "<your-email-address>";
07
08app.get("/", async (c) => {
09 const res = await fetch("https://api.resend.com/emails", {
10 method: "POST",
11 headers: {
12 "Content-Type": "application/json",
13 Authorization: `Bearer ${RESEND_API_KEY}`,
14 },
15 body: JSON.stringify({
16 from: "onboarding@resend.dev",
17 to: EMAIL_TO,
18 subject: "hello world",
19 html: "<strong>it works!</strong>",
20 }),
21 });
22
23 const data = await res.json();
24
25 return c.json(data);
26});
27
28export default app;

Set EMAIL_TO to the address that receives the message. The html field carries the message body as an HTML string, so a template literal substitutes the values that change per recipient.

The final line exports the app as an ES Modules handler, the pattern Azion recommends over the legacy Service Worker pattern. For both patterns and the move between them, refer to Migrate handler patterns in Functions.

3. Store the Resend API key as an environment variable

The handler reads RESEND_API_KEY at run time, so the key stays out of the project files and out of version control. To create the variable on your account:

azion create variables --key "RESEND_API_KEY" --value "[TOKEN VALUE]" --secret true

A variable whose key contains password, pwd, secret, key, hash, encrypted, passcode, auth, or token is sent as a secret by default. The --secret true flag repeats that default here. Give the Resend API key the smallest permission set that the send call needs.

To replace the value later, run azion update variables and deploy the project again. A change to a variable does not reach a function that is already running.

The account holds the variable, and the handler reads it by key after the deployment.

4. Deploy the project

To build the project and publish it:

azion deploy

The deployment uploads the function code, configures the application, sets the routing rules, applies the environment variables and secrets, and returns a domain in the format https://xxxxxxx.map.azionedge.net. Azion opens a browser at the Azion Console page that carries the deployment logs.

The endpoint answers on the returned domain a few minutes after propagation.

5. Send the first email

To run the handler, request the root path of the domain the deployment returned:

curl https://<your-azion-domain>/

The response body is the JSON that the Resend API returns for the request. The message arrives in the inbox of the address set in EMAIL_TO, with the subject hello world and the body it works!.

If the response carries an error from Resend, confirm that RESEND_API_KEY exists on your account and that the project was deployed after the variable was created. For a function that does not run, stops early, or writes an error to its logs, refer to Troubleshoot function execution and logs.

The endpoint now sends a transactional email on every request to its root path.

Next steps

Environment variablesThe three fields of a variable, the interface that reads one, and the CLI subcommands that manage them.Migrate handler patterns in FunctionsThe ES Modules and Service Worker handlers, their parameters, and how to move a function between them.Functions limitsThe CPU time, wall-clock time, and sub-request ceilings that one invocation runs inside.Functions best practicesWhat each part of the handler does, and the design choices that keep an invocation inside its limits.How to build with HonoThe full azion init journey for a Hono project, including the local development server.Resend APIThe fields the Resend emails endpoint accepts, and the rest of its API.