diff --git a/types/smtp-server/index.d.ts b/types/smtp-server/index.d.ts
index 3b5f320613..eaaf108777 100644
--- a/types/smtp-server/index.d.ts
+++ b/types/smtp-server/index.d.ts
@@ -1,4 +1,4 @@
-// Type definitions for smtp-server 3.4
+// Type definitions for smtp-server 3.5
 // Project: https://github.com/nodemailer/smtp-server
 // Definitions by: markisme <https://github.com/markisme>
 //                 taisiias <https://github.com/Taisiias>
@@ -12,7 +12,7 @@
 import { EventEmitter } from 'events';
 import * as net from 'net';
 import * as shared from 'nodemailer/lib/shared';
-import { Readable } from 'stream';
+import { PassThrough } from 'stream';
 import * as tls from 'tls';
 
 export type ms = number;
@@ -113,6 +113,19 @@ export interface SMTPServerSession {
     tlsOptions: tls.TlsOptions;
 }
 
+export interface SMTPServerDataStream extends PassThrough {
+    /**
+     * number of bytes read from DATA stream
+     */
+    byteLength: number;
+    /**
+     * boolean, if set to true, the message was oversized.
+     * When creating the server you can define maximum allowed message size with
+     * the size option, see RFC1870 for details.
+     */
+    sizeExceeded: boolean;
+}
+
 export interface SMTPServerEnvelope {
     /**
      * includes an address object or is set to false
@@ -270,7 +283,7 @@ export interface SMTPServerOptions extends tls.TlsOptions {
     /**
      * the callback to handle incoming messages ([see details](https://github.com/andris9/smtp-server#processing-incoming-message))
      */
-    onData?(stream: Readable, session: SMTPServerSession, callback: (err?: Error | null) => void): void;
+    onData?(stream: SMTPServerDataStream, session: SMTPServerSession, callback: (err?: Error | null) => void): void;
     /**
      * the callback that informs about closed client connection
      */
@@ -310,7 +323,7 @@ export class SMTPServer extends EventEmitter {
     /** Override this */
     onConnect(session: SMTPServerSession, callback: (err?: Error | null) => void): void;
     /** Override this */
-    onData(stream: Readable, session: SMTPServerSession, callback: (err?: Error | null) => void): void;
+    onData(stream: SMTPServerDataStream, session: SMTPServerSession, callback: (err?: Error | null) => void): void;
     /** Override this */
     onMailFrom(address: SMTPServerAddress, session: SMTPServerSession, callback: (err?: Error | null) => void): void;
     /** Override this */
diff --git a/types/smtp-server/smtp-server-tests.ts b/types/smtp-server/smtp-server-tests.ts
index 52578ad4a8..95a1338d19 100644
--- a/types/smtp-server/smtp-server-tests.ts
+++ b/types/smtp-server/smtp-server-tests.ts
@@ -1,5 +1,5 @@
 import { AddressInfo } from 'net';
-import { SMTPServer, SMTPServerAddress, SMTPServerAuthentication, SMTPServerAuthenticationResponse, SMTPServerOptions, SMTPServerSession } from 'smtp-server';
+import { SMTPServer, SMTPServerAddress, SMTPServerAuthentication, SMTPServerAuthenticationResponse, SMTPServerDataStream, SMTPServerOptions, SMTPServerSession } from 'smtp-server';
 import { Readable } from 'stream';
 
 function test_with_handlers_as_options() {
@@ -42,9 +42,14 @@ function test_with_handlers_as_options() {
         callback();
     }
 
-    function onData(stream: Readable, session: SMTPServerSession, callback: (err?: Error) => void): void {
+    function onData(stream: SMTPServerDataStream, session: SMTPServerSession, callback: (err?: Error) => void): void {
         console.log(`[${session.id}] onData started`);
 
+        if (stream.sizeExceeded) {
+            callback(new Error('Message too big'));
+            return;
+        }
+
         let messageLength = 0;
 
         stream.on('data', (chunk: Buffer) => {