feat(worker): add send_email binding support

examples/send-email/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "send-email-on-workers"
+version = "0.1.0"
+edition = "2021"
+
+[package.metadata.release]
+release = false
+
+[lib]
+crate-type = ["cdylib"]
+
+[dependencies]
+# Default feature `gethostname` pulls in a crate that doesn't build for
+# `wasm32-unknown-unknown`, so disable it. The remaining core API is enough
+# to assemble a message as long as we set `date` and `message_id` ourselves
+# (the auto-generated ones rely on `SystemTime::now()` / `gethostname`).
+mail-builder = { version = "0.4", default-features = false }
+worker.workspace = true

examples/send-email/README.md

@@ -0,0 +1,42 @@
+# Sending Email from Cloudflare Workers
+
+Demonstration of using `worker::SendEmail` to dispatch an outbound message
+through a `[[send_email]]` binding.
+
+Two paths are shown:
+
+* `GET /` — the **structured** path, using
+  [`Message::builder`](https://docs.rs/worker/latest/worker/struct.MessageBuilder.html).
+  The runtime composes the MIME body from the fields you set (`from`, `to`,
+  `subject`, `text`/`html`, attachments, etc.).
+* `GET /raw` — the **raw MIME** path, using
+  [`EmailMessage`](https://docs.rs/worker/latest/worker/struct.EmailMessage.html).
+  The MIME body is built locally with
+  [`mail-builder`](https://crates.io/crates/mail-builder) and handed verbatim
+  to the binding. Use this when you need precise control over the MIME
+  structure (custom headers, DKIM passthrough, VERP bounces, etc.).
+
+## Local development
+
+Running `wrangler dev --local` does **not** actually deliver the email. Per
+the [Cloudflare docs](https://developers.cloudflare.com/email-routing/email-workers/local-development/),
+outbound messages are simulated by writing each one to a local `.eml` file —
+the path is printed in the terminal so you can inspect the raw message.
+
+```bash
+npm install
+npm run dev
+# then, in another shell:
+curl http://localhost:8787/        # structured
+curl http://localhost:8787/raw     # raw MIME
+```
+
+## Deploying
+
+Before deploying, verify the sender and recipient addresses as documented at
+<https://developers.cloudflare.com/email-service/api/send-emails/workers-api/>,
+then:
+
+```bash
+npm run deploy
+```

examples/send-email/package.json

@@ -0,0 +1,12 @@
+{
+	"name": "send-email-on-workers",
+	"version": "0.0.0",
+	"private": true,
+	"scripts": {
+		"deploy": "cargo install worker-build ; wrangler deploy",
+		"dev": "cargo install worker-build ; wrangler dev --local"
+	},
+	"devDependencies": {
+		"wrangler": "^4.83.0"
+	}
+}

examples/send-email/src/lib.rs

@@ -0,0 +1,53 @@
+use mail_builder::MessageBuilder as MimeBuilder;
+use worker::*;
+
+const SENDER: &str = "sender@example.com";
+const RECIPIENT: &str = "recipient@example.com";
+
+#[event(fetch)]
+async fn fetch(req: Request, env: Env, _ctx: Context) -> Result<Response> {
+    let sender = env.send_email("EMAIL")?;
+
+    let result = match req.path().as_str() {
+        "/" => send_structured(&sender).await?,
+        "/raw" => send_raw_mime(&sender).await?,
+        // Don't dispatch on favicon / unknown paths — otherwise every browser
+        // tab to localhost sends a real email in `wrangler dev`.
+        _ => return Response::error("not found", 404),
+    };
+
+    Response::ok(format!("sent: {}", result.message_id))
+}
+
+async fn send_structured(sender: &SendEmail) -> Result<EmailSendResult> {
+    let email = Email::builder()
+        .from(("Sending email test", SENDER))
+        .to(RECIPIENT)
+        .subject("An email generated in a Worker")
+        .text("Congratulations, you just sent an email from a Worker.")
+        .html("<p>Congratulations, you just sent an email from a Worker.</p>")
+        .build()?;
+
+    sender.send(&email).await
+}
+
+async fn send_raw_mime(sender: &SendEmail) -> Result<EmailSendResult> {
+    // mail-builder's auto-generated `Date:` and `Message-ID:` headers rely on
+    // `SystemTime::now()` and `gethostname`, neither of which work on
+    // `wasm32-unknown-unknown`. https://github.com/stalwartlabs/mail-builder/pull/26
+    let now_ms = Date::now().as_millis();
+    let message_id = format!("{now_ms}@example.com");
+
+    let raw = MimeBuilder::new()
+        .from(("Sending email test", SENDER))
+        .to(RECIPIENT)
+        .subject("An email generated in a Worker")
+        .date((now_ms / 1000) as i64)
+        .message_id(message_id)
+        .text_body("Congratulations, you just sent an email from a Worker.")
+        .write_to_string()
+        .map_err(|e| Error::RustError(e.to_string()))?;
+
+    let message = EmailMessage::new(SENDER, RECIPIENT, &raw)?;
+    sender.send_mime(&message).await
+}

examples/send-email/wrangler.toml

@@ -0,0 +1,11 @@
+name = "send-email-on-workers"
+main = "build/index.js"
+compatibility_date = "2024-10-01"
+
+[build]
+command = "cargo install \"worker-build@^0.8\" && worker-build --release"
+# For development: use local worker-build binary
+# command = "../../target/release/worker-build --release"
+
+[[send_email]]
+name = "EMAIL"

package.json

@@ -15,7 +15,7 @@
   "homepage": "https://github.com/cloudflare/workers-rs#readme",
   "devDependencies": {
     "@types/node": "^24.0.1",
-    "miniflare": "^4.20260329.0",
+    "miniflare": "^4.20260420.0",
     "typescript": "^5.8.3",
     "uuid": "^11.1.0",
     "vitest": "^3.2.4"

test/src/lib.rs

@@ -32,6 +32,7 @@ mod rate_limit;
 mod request;
 mod router;
 mod secret_store;
+mod send_email;
 mod service;
 mod socket;
 mod sql_counter;

test/src/router.rs

@@ -1,7 +1,8 @@
 use crate::{
     alarm, analytics_engine, assets, auto_response, cache, container, counter, d1, durable, fetch,
-    form, js_snippets, kv, put_raw, queue, r2, rate_limit, request, secret_store, service, socket,
-    sql_counter, sql_iterator, user, ws, SomeSharedData, GLOBAL_SECOND_START, GLOBAL_STATE,
+    form, js_snippets, kv, put_raw, queue, r2, rate_limit, request, secret_store, send_email,
+    service, socket, sql_counter, sql_iterator, user, ws, SomeSharedData, GLOBAL_SECOND_START,
+    GLOBAL_STATE,
 };
 #[cfg(feature = "http")]
 use std::convert::TryInto;
@@ -239,6 +240,7 @@ macro_rules! add_routes (
     add_route!($obj, get, format_route!("/rate-limit/key/{}", "key"), rate_limit::handle_rate_limit_with_key);
     add_route!($obj, get, "/rate-limit/bulk-test", rate_limit::handle_rate_limit_bulk_test);
     add_route!($obj, get, "/rate-limit/reset", rate_limit::handle_rate_limit_reset);
+    add_route!($obj, get, "/send-email", send_email::handle_send_email);
 });
 
 #[cfg(feature = "http")]