diff --git a/packages/mail/src/mail.d.ts b/packages/mail/src/mail.d.ts
index fa827498..0930ddc0 100644
--- a/packages/mail/src/mail.d.ts
+++ b/packages/mail/src/mail.d.ts
@@ -32,12 +32,12 @@ declare class MailService {
   /**
    * Send email
    */
-  send(data: MailDataRequired | MailDataRequired[], isMultiple?: boolean, cb?: (err: Error | ResponseError, result: [ClientResponse, {}]) => void): Promise<[ClientResponse, {}]>;
+  send(data: MailDataRequired | MailDataRequired[], isMultiple?: boolean, cb?: (err: Error | ResponseError | null, result: [ClientResponse, {}]) => void): Promise<[ClientResponse, {}]>;
 
   /**
    * Send multiple emails (shortcut)
    */
-  sendMultiple(data: MailDataRequired, cb?: (error: Error | ResponseError, result: [ClientResponse, {}]) => void): Promise<[ClientResponse, {}]>;
+  sendMultiple(data: MailDataRequired, cb?: (error: Error | ResponseError | null, result: [ClientResponse, {}]) => void): Promise<[ClientResponse, {}]>;
 }
 
 declare const mail: MailService;
diff --git a/packages/mail/src/mail.spec.js b/packages/mail/src/mail.spec.js
index 09698baa..623161b6 100644
--- a/packages/mail/src/mail.spec.js
+++ b/packages/mail/src/mail.spec.js
@@ -47,6 +47,21 @@ describe('sgMail.send()', () => {
       });
   });
 
+  it('should not be null(error) in callBack function for unsuccessfully delivered emails', (done) => {
+    sgMail.send({}, false, (error, response) => {
+      expect(error).to.not.be.null();
+      done();
+    });
+  });
+
+  it('should be null(error) in callBack function for successfully delivered emails', (done) => {
+    sgClient.setDefaultHeader('X-Mock', 202);
+    sgMail.send(data, false, (error, response) => {
+      expect(error).to.be.null();
+      done();
+    });
+  });
+
   it('should throw an error if callback is not a function', () => {
     return expect(function() {
       sgMail.send(data, false, {});