diff --git a/src/handlers.js b/src/handlers.js
index 9b3484c..18f4b3e 100644
--- a/src/handlers.js
+++ b/src/handlers.js
@@ -3,10 +3,20 @@ import { params } from './params';
 
 const postApiNotify = async ({ env, request }) => {
   const { errors = [], ...rest } = await params(request);
-  await mail({
-    env,
-    ...rest,
-  });
+  try {
+    await mail({
+      env,
+      ...rest,
+    });
+  } catch (e) {
+    return new Response(JSON.stringify({ error: e.message }), {
+      headers: {
+        'Content-Type': 'application/json',
+      },
+      status: 400,
+    });
+  }
+
   const response = {
     ...rest,
     ...(errors.length && { errors }),
diff --git a/src/handlers.spec.js b/src/handlers.spec.js
index 49c71ad..09486c1 100644
--- a/src/handlers.spec.js
+++ b/src/handlers.spec.js
@@ -5,10 +5,10 @@ import { payload } from './mail';
 
 describe('handlers', () => {
   beforeAll(() => {
-    global.fetch = vi.fn();
+    global.fetch = vi.fn().mockImplementation(() => new Response());
   });
 
-  it('400', async () => {
+  it('400 if missing required parameters', async () => {
     const request = new Request('http://example.com/api/notify', {
       method: 'POST',
       headers: {
@@ -23,6 +23,37 @@ describe('handlers', () => {
     );
   });
 
+  it('400 if invalid to: address', async () => {
+    const request = new Request('http://example.com/api/notify', {
+      method: 'POST',
+      body: JSON.stringify({ to: 'invalid email', subject: 'valid subject' }),
+      headers: {
+        'Content-Type': 'application/json',
+      },
+    });
+
+    global.fetch.mockImplementationOnce(() => {
+      return new Response(
+        JSON.stringify({
+          errors: [
+            {
+              message: 'Does not contain a valid address.',
+              field: 'personalizations.0.to.0.email',
+              help: 'http://sendgrid.com/docs/API_Reference/Web_API_v3/Mail/errors.html#message.personalizations.to',
+            },
+          ],
+        }),
+        { status: 400 },
+      );
+    });
+
+    const response = await handler({ request });
+    expect(await response.status).toEqual(400);
+    expect(await response.json()).toEqual(
+      expect.objectContaining({ error: 'Does not contain a valid address.' }),
+    );
+  });
+
   it('404', async () => {
     const request = new Request('http://example.com/undefined-route');
     const response = await handler({ request });
diff --git a/src/mail.js b/src/mail.js
index d18f3f4..c11db55 100644
--- a/src/mail.js
+++ b/src/mail.js
@@ -23,7 +23,7 @@ export const payload = ({ subject, to, type, body, attachments }) => ({
  * @returns Promise<void>
  */
 export const mail = async ({ env = {}, ...rest }) => {
-  await fetch('https://api.sendgrid.com/v3/mail/send', {
+  const response = await fetch('https://api.sendgrid.com/v3/mail/send', {
     headers: {
       Authorization: `Bearer ${env.SENDGRID_API_KEY}`,
       'Content-Type': 'application/json',
@@ -31,4 +31,7 @@ export const mail = async ({ env = {}, ...rest }) => {
     method: 'POST',
     body: JSON.stringify(payload({ ...rest })),
   });
+  if (response.status > 299) {
+    throw new Error((await response.json()).errors[0].message);
+  }
 };