diff --git a/src/__tests__/lib/mailtrap-client.test.ts b/src/__tests__/lib/mailtrap-client.test.ts
index f92496a..7a976e6 100644
--- a/src/__tests__/lib/mailtrap-client.test.ts
+++ b/src/__tests__/lib/mailtrap-client.test.ts
@@ -15,7 +15,7 @@ import TemplatesBaseAPI from "../../lib/api/Templates";
 
 const { ERRORS, CLIENT_SETTINGS } = CONFIG;
 const { TESTING_ENDPOINT, BULK_ENDPOINT, SENDING_ENDPOINT } = CLIENT_SETTINGS;
-const { TEST_INBOX_ID_MISSING, ACCOUNT_ID_MISSING, BULK_SANDBOX_INCOMPATIBLE } =
+const { ACCOUNT_ID_MISSING, BULK_SANDBOX_INCOMPATIBLE, TEST_INBOX_ID_MISSING } =
   ERRORS;
 
 describe("lib/mailtrap-client: ", () => {
@@ -331,7 +331,49 @@ describe("lib/mailtrap-client: ", () => {
       }
     });
 
+    it("throws MailtrapError(TEST_INBOX_ID_MISSING) when sending in sandbox mode without testInboxId", async () => {
+      const client = new MailtrapClient({
+        token: "MY_API_TOKEN",
+        sandbox: true,
+        accountId: 123,
+      });
+
+      await expect(
+        client.send({
+          from: { email: "a@b.com", name: "Sender" },
+          to: [{ email: "c@d.com" }],
+          subject: "Test",
+          text: "Body",
+        })
+      ).rejects.toEqual(new MailtrapError(TEST_INBOX_ID_MISSING));
+    });
+
     describe("batch sending:", () => {
+      it("throws MailtrapError(TEST_INBOX_ID_MISSING) when batch sending in sandbox mode without testInboxId", async () => {
+        const client = new MailtrapClient({
+          token: "MY_API_TOKEN",
+          sandbox: true,
+          accountId: 123,
+        });
+
+        const batchData = {
+          base: {
+            from: { email: "a@b.com", name: "Sender" },
+            subject: "Test",
+            text: "Body",
+          },
+          requests: [
+            {
+              to: [{ email: "c@d.com" }],
+            },
+          ],
+        };
+
+        await expect(client.batchSend(batchData)).rejects.toEqual(
+          new MailtrapError(TEST_INBOX_ID_MISSING)
+        );
+      });
+
       it("rejects with Mailtrap error when bulk and sandbox modes are used together", async () => {
         const batchClient = new MailtrapClient({
           token: "MY_API_TOKEN",
@@ -664,7 +706,7 @@ describe("lib/mailtrap-client: ", () => {
   });
 
   describe("get testing(): ", () => {
-    it("rejects with Mailtrap error, when `testInboxId` is missing.", () => {
+    it("rejects with Mailtrap error, when `accountId` is missing.", () => {
       const client = new MailtrapClient({
         token: "MY_API_TOKEN",
       });
@@ -674,30 +716,25 @@ describe("lib/mailtrap-client: ", () => {
       try {
         client.testing;
       } catch (error) {
-        expect(error).toEqual(new MailtrapError(TEST_INBOX_ID_MISSING));
+        expect(error).toEqual(new MailtrapError(ACCOUNT_ID_MISSING));
       }
     });
 
-    it("rejects with Mailtrap error, when `accountId` is missing.", () => {
+    it("returns testing API object when accountId is provided, even without testInboxId", () => {
       const client = new MailtrapClient({
         token: "MY_API_TOKEN",
-        testInboxId: 5,
+        accountId: 123,
+        // testInboxId is intentionally omitted
       });
-
       expect.assertions(1);
 
-      try {
-        client.testing;
-      } catch (error) {
-        expect(error).toEqual(new MailtrapError(ACCOUNT_ID_MISSING));
-      }
+      const testingClient = client.testing;
+      expect(testingClient).toBeInstanceOf(TestingAPI);
     });
 
     it("returns testing API object, console warn is called twice.", () => {
       const client = new MailtrapClient({
         token: "MY_API_TOKEN",
-        sandbox: true,
-        testInboxId: 10,
         accountId: 10,
       });
       expect.assertions(1);
diff --git a/src/lib/MailtrapClient.ts b/src/lib/MailtrapClient.ts
index 280342d..8b17e5a 100644
--- a/src/lib/MailtrapClient.ts
+++ b/src/lib/MailtrapClient.ts
@@ -32,7 +32,7 @@ const {
   TESTING_ENDPOINT,
   BULK_ENDPOINT,
 } = CLIENT_SETTINGS;
-const { TEST_INBOX_ID_MISSING, ACCOUNT_ID_MISSING, BULK_SANDBOX_INCOMPATIBLE } =
+const { ACCOUNT_ID_MISSING, BULK_SANDBOX_INCOMPATIBLE, TEST_INBOX_ID_MISSING } =
   ERRORS;
 
 /**
@@ -93,13 +93,18 @@ export default class MailtrapClient {
   }
 
   /**
-   * Getter for Testing API. Warns if some of the required keys are missing.
+   * Validates that test inbox ID is present, throws MailtrapError if missing.
    */
-  get testing() {
-    if (!this.testInboxId) {
+  private validateTestInboxIdPresence(): void {
+    if (this.sandbox && !this.testInboxId) {
       throw new MailtrapError(TEST_INBOX_ID_MISSING);
     }
+  }
 
+  /**
+   * Getter for Testing API. Warns if some of the required keys are missing.
+   */
+  get testing() {
     this.validateAccountIdPresence();
 
     return new TestingAPI(this.axios, this.accountId);
@@ -132,6 +137,9 @@ export default class MailtrapClient {
     return new ContactListsBaseAPI(this.axios, this.accountId);
   }
 
+  /**
+   * Getter for Templates API.
+   */
   get templates() {
     this.validateAccountIdPresence();
 
@@ -164,8 +172,11 @@ export default class MailtrapClient {
    */
   public async send(mail: Mail): Promise<SendResponse> {
     const host = this.determineHost();
+
+    this.validateTestInboxIdPresence();
+
     const url = `${host}/api/send${
-      this.testInboxId ? `/${this.testInboxId}` : ""
+      this.sandbox && this.testInboxId ? `/${this.testInboxId}` : ""
     }`;
     const preparedMail = encodeMailBuffers(mail);
 
@@ -181,9 +192,13 @@ export default class MailtrapClient {
   ): Promise<BatchSendResponse> {
     const { requests, base } = request;
     const host = this.determineHost();
-    const ifSandbox =
+
+    this.validateTestInboxIdPresence();
+
+    const sandbox =
       this.sandbox && this.testInboxId ? `/${this.testInboxId}` : "";
-    const url = `${host}/api/batch${ifSandbox}`;
+
+    const url = `${host}/api/batch${sandbox}`;
 
     const preparedBase = base ? encodeMailBuffers(base) : undefined;
     const preparedRequests = requests.map((singleRequest) =>