diff --git a/README.md b/README.md
index e1e2eb72..ec6ec412 100644
--- a/README.md
+++ b/README.md
@@ -19,7 +19,7 @@ https://github.com/cloudflare/agentic-inbox/issues/4#issuecomment-4269118513
 
 ### To set up
 
-1. Deploy to Cloudflare. The deploy flow will automatically provision R2, Durable Objects, and Workers AI. You'll be prompted for **DOMAINS**, which is the domain (yourdomain.com) you want to receive emails for (email@yourdomain.com).
+1. Deploy to Cloudflare. The deploy flow will automatically provision R2, Durable Objects, and Workers AI. You'll be prompted for **DOMAINS**, which is the domain (yourdomain.com) you want to receive emails for (email@yourdomain.com). To serve more than one domain from a single instance, pass a comma-separated list (e.g. `yourdomain.com,anotherdomain.com`) — see [Using multiple domains](#using-multiple-domains).
 
      [![Deploy to Cloudflare](https://deploy.workers.cloudflare.com/button)](https://deploy.workers.cloudflare.com/?url=https://github.com/cloudflare/agentic-inbox)
 
@@ -59,9 +59,27 @@ npm run dev
 
 ### Configuration
 
-1. Set your domain in `wrangler.jsonc`
+1. Set your domain (or domains) in `wrangler.jsonc` via the `DOMAINS` var
 2. Create an R2 bucket named `agentic-inbox`: `wrangler r2 bucket create agentic-inbox`
 
+### Using multiple domains
+
+A single instance can serve multiple domains. Set `DOMAINS` to a comma-separated list:
+
+```jsonc
+"DOMAINS": "example.com,another.com"
+```
+
+Then, for **each** domain:
+
+- Add a catch-all [Email Routing](https://developers.cloudflare.com/email-routing/) rule that forwards to this Worker (for receiving)
+- Verify the domain for [Email Service](https://developers.cloudflare.com/email-service/) (for sending)
+
+Notes:
+
+- The **New Mailbox** dialog shows a domain picker automatically once more than one domain is configured; mailbox creation is restricted to the configured domains.
+- If you set `EMAIL_ADDRESSES` to restrict mailbox creation, it may list addresses across any of the configured domains (e.g. `["hello@example.com", "hi@another.com"]`).
+
 ### Deploy
 
 ```bash
diff --git a/package.json b/package.json
index 9d6f0272..311d4064 100644
--- a/package.json
+++ b/package.json
@@ -8,7 +8,7 @@
 		"products": ["Workers", "Durable Objects", "R2", "Workers AI"],
 		"bindings": {
 			"DOMAINS": {
-				"description": "Your domain with [Email Routing](https://developers.cloudflare.com/email-routing/) enabled (e.g. `example.com`). After deploying, create a catch-all Email Routing rule pointing to this Worker."
+				"description": "Your domain with [Email Routing](https://developers.cloudflare.com/email-routing/) enabled (e.g. `example.com`). For multiple domains, pass a comma-separated list (e.g. `example.com,another.com`). After deploying, create a catch-all Email Routing rule pointing to this Worker for each domain."
 			}
 		}
 	},
diff --git a/workers/index.ts b/workers/index.ts
index fd3359ce..4301fe49 100644
--- a/workers/index.ts
+++ b/workers/index.ts
@@ -50,6 +50,12 @@ function slugify(text: string) { // can return "" for non-alphanumeric input
 		.replace(/--+/g, "-").replace(/^-+/, "").replace(/-+$/, "");
 }
 
+// Parse the comma-separated DOMAINS var into a trimmed, non-empty list.
+// Supports multiple domains on one instance, e.g. "example.com,another.com".
+function parseDomains(raw: string | undefined): string[] {
+	return (raw || "").split(",").map((d) => d.trim()).filter(Boolean);
+}
+
 function intQuery(c: AppContext, key: string): number | undefined {
 	const v = c.req.query(key);
 	if (!v) return undefined;
@@ -86,8 +92,7 @@ app.use("/api/v1/mailboxes/:mailboxId/*", requireMailbox);
 // -- Config ---------------------------------------------------------
 
 app.get("/api/v1/config", (c) => {
-	const domainsRaw = c.env.DOMAINS || "";
-	const domains = domainsRaw.split(",").map((d) => d.trim()).filter(Boolean);
+	const domains = parseDomains(c.env.DOMAINS);
 	const emailAddresses = c.env.EMAIL_ADDRESSES ?? [];
 	return c.json({ domains, emailAddresses });
 });
@@ -103,9 +108,18 @@ app.post("/api/v1/mailboxes", async (c) => {
 	const { name, settings, email: rawEmail } = CreateMailboxBody.parse(await c.req.json());
 	const email = rawEmail.toLowerCase();
 	const allowedAddresses = (c.env.EMAIL_ADDRESSES ?? []) as string[];
-	if (allowedAddresses.length > 0 && !allowedAddresses.map((a) => a.toLowerCase()).includes(email)) {
+	const isExplicitlyAllowed = allowedAddresses.map((a) => a.toLowerCase()).includes(email);
+	if (allowedAddresses.length > 0 && !isExplicitlyAllowed) {
 		return c.json({ error: "Mailbox creation is restricted to configured EMAIL_ADDRESSES" }, 403);
 	}
+	// When DOMAINS is configured, mailboxes must be on one of those domains — this mirrors the
+	// front-end domain picker. Explicit EMAIL_ADDRESSES entries bypass the check (they are the
+	// authoritative allow-list and may legitimately span domains).
+	const domains = parseDomains(c.env.DOMAINS);
+	const domain = email.split("@")[1];
+	if (!isExplicitlyAllowed && domains.length > 0 && (!domain || !domains.some((d) => d.toLowerCase() === domain))) {
+		return c.json({ error: "Mailbox domain must be one of the configured DOMAINS" }, 400);
+	}
 	const key = `mailboxes/${email}.json`;
 	if (await c.env.BUCKET.head(key)) return c.json({ error: "Mailbox already exists" }, 409);
 	const defaultSettings = { fromName: name, forwarding: { enabled: false, email: "" }, signature: { enabled: false, text: "" }, autoReply: { enabled: false, subject: "", message: "" } };
diff --git a/wrangler.jsonc b/wrangler.jsonc
index 53a65cd0..98300c5b 100644
--- a/wrangler.jsonc
+++ b/wrangler.jsonc
@@ -13,7 +13,13 @@
 		// Production deploys must also define POLICY_AUD and TEAM_DOMAIN.
 		// TEAM_DOMAIN may be the base Access URL or the full /cdn-cgi/access/certs URL.
 		// The worker now fails closed outside local development if Access is not configured.
+		//
+		// DOMAINS accepts a single domain or a comma-separated list to serve multiple
+		// domains from one instance, e.g. "example.com,another.com". Each domain needs its
+		// own Email Routing catch-all rule, and must be verified for outbound sending.
 		"DOMAINS": "example.com",
+		// EMAIL_ADDRESSES optionally restricts mailbox creation to specific addresses, and
+		// may span the configured domains, e.g. ["hello@example.com", "hi@another.com"].
 		"EMAIL_ADDRESSES": []
 	},
 	"send_email": [