diff --git a/lib/interfaces/mailer-options.interface.ts b/lib/interfaces/mailer-options.interface.ts
index a5d14429..cbdf9fc9 100644
--- a/lib/interfaces/mailer-options.interface.ts
+++ b/lib/interfaces/mailer-options.interface.ts
@@ -33,7 +33,7 @@ export interface MailerOptions {
   defaults?: Options;
   transport?: TransportType;
   transports?: {
-    [name: string]: SMTPTransport | SMTPTransport.Options | string;
+    [name: string]: TransportType;
   };
   template?: {
     dir?: string;
diff --git a/lib/mailer.service.ts b/lib/mailer.service.ts
index 9c5c35d5..dc9e6d1c 100644
--- a/lib/mailer.service.ts
+++ b/lib/mailer.service.ts
@@ -2,7 +2,6 @@
 import { get, defaultsDeep } from 'lodash';
 import { Injectable, Inject, Optional, Logger } from '@nestjs/common';
 import { SentMessageInfo, Transporter } from 'nodemailer';
-import * as smtpTransport from 'nodemailer/lib/smtp-transport';
 
 /** Constants **/
 import {
@@ -11,7 +10,7 @@ import {
 } from './constants/mailer.constant';
 
 /** Interfaces **/
-import { MailerOptions } from './interfaces/mailer-options.interface';
+import { MailerOptions, TransportType } from './interfaces/mailer-options.interface';
 import { TemplateAdapter } from './interfaces/template-adapter.interface';
 import { ISendMailOptions } from './interfaces/send-mail-options.interface';
 import { MailerTransportFactory as IMailerTransportFactory } from './interfaces/mailer-transport-factory.interface';
@@ -106,7 +105,7 @@ export class MailerService {
     }
   }
 
-  private createTransporter(config: string | smtpTransport | smtpTransport.Options, name?: string): Transporter {
+  private createTransporter(config: TransportType, name?: string): Transporter {
     const transporter = this.transportFactory.createTransport(config);
     if (this.mailerOptions.verifyTransporters) this.verifyTransporter(transporter, name);
     this.initTemplateAdapter(this.templateAdapter, transporter);
@@ -168,7 +167,7 @@ export class MailerService {
     }
   }
 
-  addTransporter(transporterName: string, config: string | smtpTransport | smtpTransport.Options): string {
+  addTransporter(transporterName: string, config: TransportType): string {
     this.transporters.set(
       transporterName,
       this.transportFactory.createTransport(config),