diff --git a/.env.example b/.env.example
index 930dbab..2121e02 100644
--- a/.env.example
+++ b/.env.example
@@ -8,3 +8,9 @@ SMTP_PORT=587
 SMTP_USER=resend
 SMTP_PASSWORD=
 MAIL_FROM=send@read.atanasov.fi
+
+# Comma-separated list of allowed destination email domains. Anything that
+# doesn't match (suffix-wise, so subdomains like free.kindle.com count) is
+# rejected with a 400. This prevents the app from being abused as an open
+# email relay to arbitrary addresses.
+ALLOWED_READER_DOMAINS=kindle.com,pbsync.com
diff --git a/README.md b/README.md
index c3261ce..5e59217 100644
--- a/README.md
+++ b/README.md
@@ -20,15 +20,16 @@ Two ways to send something to your e-reader:
 
 ## Supported devices
 
-Any device that accepts files via email works — the app doesn't restrict by domain.
+The destination email domain is checked against an allowlist (default: `kindle.com`, `pbsync.com`) to prevent the app being used as an open email relay. Subdomains count, so `free.kindle.com` works too.
 
 | Device | Email pattern |
 |---|---|
-| Amazon Kindle | `*@kindle.com` |
+| Amazon Kindle | `*@kindle.com` (or `*.kindle.com` subdomains) |
 | PocketBook | `*@pbsync.com` |
-| Anything else | whatever address your device gives you |
 
-You'll need to add the **sender** address (the Gmail account this app sends from) to your device's approved-senders list — that's a one-time per-device setup.
+You can extend the list via the `ALLOWED_READER_DOMAINS` env var (see below).
+
+You'll also need to add the **sender** address to your device's approved-senders list — one-time per device.
 
 ## Prerequisites
 
@@ -63,6 +64,7 @@ npm run start
 | `SMTP_USER` | yes | — | For Resend, literally the string `resend` |
 | `SMTP_PASSWORD` | yes | — | For Resend, your API key (`re_…`) |
 | `MAIL_FROM` | yes | — | Sender address, e.g. `read@atanasov.fi`. Must be from a verified domain on your provider. |
+| `ALLOWED_READER_DOMAINS` | no | `kindle.com,pbsync.com` | Comma-separated allowlist of destination email domains. Suffix-matching, so subdomains are allowed. |
 | `PORT` | no | `3000` | Listen port |
 
 ## Resend setup (quick)
diff --git a/public/index.html b/public/index.html
index 758c190..17f3a86 100644
--- a/public/index.html
+++ b/public/index.html
@@ -260,7 +260,12 @@
 
           <h3>notes:</h3>
           <ul>
-            <li>any email works — domain isn't checked, use whatever your device gave you</li>
+            <li>
+              only <code>@kindle.com</code> and <code>@pbsync.com</code>
+              destination addresses are accepted (subdomains too, e.g.
+              <code>free.kindle.com</code>). This is to prevent the form being
+              abused as an open email relay.
+            </li>
             <li>article should be publicly accessible (no paywalls / login required)</li>
             <li>uploaded files are passed through as-is — your device decides what formats it accepts (EPUB, PDF, MOBI, etc.)</li>
             <li>max file size: 25 MB</li>
diff --git a/src/app.ts b/src/app.ts
index 61681ab..620f25e 100644
--- a/src/app.ts
+++ b/src/app.ts
@@ -20,6 +20,13 @@ const SMTP_USER = process.env.SMTP_USER || "";
 const SMTP_PASSWORD = process.env.SMTP_PASSWORD || "";
 const MAIL_FROM = process.env.MAIL_FROM || "";
 
+const ALLOWED_READER_DOMAINS = (
+  process.env.ALLOWED_READER_DOMAINS || "kindle.com,pbsync.com"
+)
+  .split(",")
+  .map((d) => d.trim().toLowerCase())
+  .filter(Boolean);
+
 if (!SMTP_USER || !SMTP_PASSWORD || !MAIL_FROM) {
   console.warn(
     "Warning: SMTP_USER, SMTP_PASSWORD, or MAIL_FROM not set. Outgoing mail will fail until they are configured.",
@@ -184,6 +191,23 @@ async function sendToReader(
 
 const emailShape = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
 
+function isAllowedReaderEmail(email: string): boolean {
+  if (!emailShape.test(email)) return false;
+  const at = email.lastIndexOf("@");
+  if (at < 0) return false;
+  const domain = email.slice(at + 1).toLowerCase();
+  return ALLOWED_READER_DOMAINS.some(
+    (d) => domain === d || domain.endsWith("." + d),
+  );
+}
+
+function readerDomainsHumanReadable(): string {
+  if (ALLOWED_READER_DOMAINS.length === 0) return "(none)";
+  if (ALLOWED_READER_DOMAINS.length === 1) return `@${ALLOWED_READER_DOMAINS[0]}`;
+  const prefixed = ALLOWED_READER_DOMAINS.map((d) => `@${d}`);
+  return prefixed.slice(0, -1).join(", ") + " or " + prefixed[prefixed.length - 1];
+}
+
 app.post(
   "/send-article",
   sendLimiter,
@@ -198,8 +222,10 @@ app.post(
       res.status(400).json({ error: "No e-reader email provided" });
       return;
     }
-    if (!emailShape.test(readerEmail)) {
-      res.status(400).json({ error: "Invalid e-reader email address" });
+    if (!isAllowedReaderEmail(readerEmail)) {
+      res.status(400).json({
+        error: `E-reader email must end in ${readerDomainsHumanReadable()}.`,
+      });
       return;
     }
 
@@ -267,9 +293,11 @@ app.post(
       res.status(400).json({ error: "No e-reader email provided" });
       return;
     }
-    if (!emailShape.test(readerEmail)) {
+    if (!isAllowedReaderEmail(readerEmail)) {
       cleanup();
-      res.status(400).json({ error: "Invalid e-reader email address" });
+      res.status(400).json({
+        error: `E-reader email must end in ${readerDomainsHumanReadable()}.`,
+      });
       return;
     }