diff --git a/amber/src/main/scala/org/apache/texera/web/resource/GmailResource.scala b/amber/src/main/scala/org/apache/texera/web/resource/GmailResource.scala
index f06f1f92102..ab91c9ad437 100644
--- a/amber/src/main/scala/org/apache/texera/web/resource/GmailResource.scala
+++ b/amber/src/main/scala/org/apache/texera/web/resource/GmailResource.scala
@@ -33,6 +33,7 @@ import javax.annotation.security.RolesAllowed
 import javax.mail.internet.{InternetAddress, MimeMessage}
 import javax.mail.{Message, PasswordAuthentication, Session, Transport}
 import javax.ws.rs._
+import javax.ws.rs.core.Response
 import scala.util.{Failure, Success, Try}
 
 case class EmailMessage(
@@ -146,7 +147,13 @@ class GmailResource {
   @Path("/send")
   def sendEmailRequest(emailMessage: EmailMessage, @Auth user: SessionUser): Unit = {
     val recipientEmail = if (emailMessage.receiver.isEmpty) user.getEmail else emailMessage.receiver
-    sendEmail(emailMessage, recipientEmail)
+    sendEmail(emailMessage, recipientEmail) match {
+      case Right(_) => ()
+      case Left("Invalid email format") =>
+        throw new BadRequestException("Invalid email format")
+      case Left(error) =>
+        throw new WebApplicationException(error, Response.Status.BAD_GATEWAY)
+    }
   }
 
   @GET
diff --git a/amber/src/test/scala/org/apache/texera/web/resource/GmailResourceSpec.scala b/amber/src/test/scala/org/apache/texera/web/resource/GmailResourceSpec.scala
new file mode 100644
index 00000000000..868b0e34cda
--- /dev/null
+++ b/amber/src/test/scala/org/apache/texera/web/resource/GmailResourceSpec.scala
@@ -0,0 +1,74 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.texera.web.resource
+
+import org.apache.texera.auth.SessionUser
+import org.apache.texera.dao.jooq.generated.enums.UserRoleEnum
+import org.apache.texera.dao.jooq.generated.tables.pojos.User
+import org.scalatest.flatspec.AnyFlatSpec
+
+import javax.ws.rs.{BadRequestException, WebApplicationException}
+
+class GmailResourceSpec extends AnyFlatSpec {
+
+  private def newSessionUser(): SessionUser = {
+    val user = new User
+    user.setUid(Integer.valueOf(1))
+    user.setName("test")
+    user.setRole(UserRoleEnum.REGULAR)
+    user.setEmail("test@example.com")
+    new SessionUser(user)
+  }
+
+  it should "throw BadRequestException (HTTP 400) when the receiver fails email-format validation" in {
+    val resource = new GmailResource()
+    val msg = EmailMessage(
+      receiver = "not-a-valid-email",
+      subject = "subj",
+      content = "body"
+    )
+    val ex = intercept[BadRequestException] {
+      resource.sendEmailRequest(msg, newSessionUser())
+    }
+    assert(ex.getResponse.getStatus == 400)
+  }
+
+  it should "throw WebApplicationException with HTTP 502 when sendEmail fails for a non-validation reason" in {
+    // In the test environment `UserSystemConfig.gmail` defaults to "", so
+    // `createMimeMessage`'s `new InternetAddress(senderGmail)` raises an
+    // `AddressException` deterministically — without any network or SMTP
+    // server contact — and `sendEmail` returns `Left("Failed to send email:
+    // ...")`. The resource then maps that `Left` to a 502 BadGateway.
+    val resource = new GmailResource()
+    val msg = EmailMessage(
+      receiver = "valid@example.com",
+      subject = "subj",
+      content = "body"
+    )
+    val ex = intercept[WebApplicationException] {
+      resource.sendEmailRequest(msg, newSessionUser())
+    }
+    assert(
+      !ex.isInstanceOf[BadRequestException],
+      s"expected non-validation failure, but got BadRequestException: ${ex.getMessage}"
+    )
+    assert(ex.getResponse.getStatus == 502)
+  }
+}
diff --git a/frontend/src/app/common/service/gmail/gmail.service.spec.ts b/frontend/src/app/common/service/gmail/gmail.service.spec.ts
new file mode 100644
index 00000000000..fd4ce18ca72
--- /dev/null
+++ b/frontend/src/app/common/service/gmail/gmail.service.spec.ts
@@ -0,0 +1,66 @@
+/**
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+import { TestBed } from "@angular/core/testing";
+import { HttpClientTestingModule, HttpTestingController } from "@angular/common/http/testing";
+import { GmailService } from "./gmail.service";
+import { NotificationService } from "../notification/notification.service";
+
+describe("GmailService", () => {
+  let service: GmailService;
+  let httpTestingController: HttpTestingController;
+  let notificationSpy: { success: ReturnType<typeof vi.fn>; error: ReturnType<typeof vi.fn> };
+
+  beforeEach(() => {
+    notificationSpy = { success: vi.fn(), error: vi.fn() };
+    TestBed.configureTestingModule({
+      imports: [HttpClientTestingModule],
+      providers: [GmailService, { provide: NotificationService, useValue: notificationSpy }],
+    });
+    service = TestBed.inject(GmailService);
+    httpTestingController = TestBed.inject(HttpTestingController);
+  });
+
+  afterEach(() => {
+    httpTestingController.verify();
+  });
+
+  it("should show a success toast when the backend accepts the send request", () => {
+    service.sendEmail("subj", "body", "to@example.com");
+
+    const req = httpTestingController.expectOne(r => r.url.endsWith("/gmail/send") && r.method === "PUT");
+    req.flush(null);
+
+    expect(notificationSpy.success).toHaveBeenCalledWith("Email sent successfully");
+    expect(notificationSpy.error).not.toHaveBeenCalled();
+  });
+
+  it("should show an error toast when the backend returns an HTTP error (e.g. SMTP failure)", () => {
+    service.sendEmail("subj", "body", "to@example.com");
+
+    const req = httpTestingController.expectOne(r => r.url.endsWith("/gmail/send") && r.method === "PUT");
+    req.flush("Failed to send email: 535-5.7.8 Username and Password not accepted", {
+      status: 502,
+      statusText: "Bad Gateway",
+    });
+
+    expect(notificationSpy.error).toHaveBeenCalledWith("Failed to send email. Please try again or contact admin.");
+    expect(notificationSpy.success).not.toHaveBeenCalled();
+  });
+});
diff --git a/frontend/src/app/common/service/gmail/gmail.service.ts b/frontend/src/app/common/service/gmail/gmail.service.ts
index 925e7e3a39d..3dfec89a064 100644
--- a/frontend/src/app/common/service/gmail/gmail.service.ts
+++ b/frontend/src/app/common/service/gmail/gmail.service.ts
@@ -42,7 +42,8 @@ export class GmailService {
         next: () => this.notificationService.success("Email sent successfully"),
         error: (error: unknown) => {
           if (error instanceof HttpErrorResponse) {
-            console.error(error.error);
+            this.notificationService.error("Failed to send email. Please try again or contact admin.");
+            console.error("Send email error:", error.error);
           }
         },
       });