Dev transports

During local development you usually want to see the emails your app sends without an SMTP server or a sandbox account. v4 ships two nodemailer transports for that: FileTransport writes every email to an .eml file (open it with any mail client) plus an .html file for the browser, and LogTransport prints a summary to the Nest logger. Both send nothing and work with templates, mail classes and every other feature.

FileTransport

import { Module } from '@nestjs/common';
import { FileTransport, MailerModule } from '@nestjs-modules/mailer';

@Module({
  imports: [
    MailerModule.forRoot({
      transport: new FileTransport({ directory: 'var/mail' }),
      defaults: { from: 'App <app@example.com>' },
    }),
  ],
})
export class AppModule {}

Option	Default	Description
directory	'var/mail'	Where files are written, resolved from the working directory and created when missing
html	true	Also write the HTML body next to the .eml
log	true	Log the path of every file written

Files are named after the send time followed by the Message-ID stripped to letters, digits and dashes (for example var/mail/2026-10-23T09-15-00-000Z-a1b2c3d4examplecom.eml). The send result includes the file path.

Add the directory to .gitignore.

LogTransport

import { Logger, Module } from '@nestjs/common';
import { LogTransport, MailerModule } from '@nestjs-modules/mailer';

@Module({
  imports: [
    MailerModule.forRoot({
      transport: new LogTransport({ body: true, logger: new Logger('Mail') }),
    }),
  ],
})
export class AppModule {}

Option	Default	Description
body	false	Log the text body too
logger	new Logger('LogTransport')	Any Nest LoggerService

Each email logs its subject, sender, envelope recipients and attachment names.

Production warning

These transports (and TestMailbox) never deliver email. If one is configured as transport or in transports while NODE_ENV is production, the mailer logs a warning at startup naming the option, since that is almost always a configuration mistake. Custom transports can opt into the same check by setting the exported LOCAL_TRANSPORT symbol:

import { LOCAL_TRANSPORT } from '@nestjs-modules/mailer';

export class SlackPreviewTransport {
  readonly name = 'SlackPreviewTransport';
  readonly version = '1.0.0';
  readonly [LOCAL_TRANSPORT] = true;

  send(mail: unknown, callback: (err: Error | null, info?: unknown) => void) {
    // post a summary to Slack instead of delivering…
    callback(null, { messageId: 'preview' });
  }
}

Choosing per environment

import { Module } from '@nestjs/common';
import { ConfigModule, ConfigService } from '@nestjs/config';
import { FileTransport, MailerModule } from '@nestjs-modules/mailer';

@Module({
  imports: [
    MailerModule.forRootAsync({
      imports: [ConfigModule],
      inject: [ConfigService],
      useFactory: (config: ConfigService) => ({
        transport:
          config.get('NODE_ENV') === 'production'
            ? config.get('SMTP_URL')
            : new FileTransport(),
      }),
    }),
  ],
})
export class AppModule {}

Related

• Email previews render emails in the browser on demand, without sending.
• The preview option opens each sent email in the browser through preview-email.
• For tests, use TestMailbox instead: it keeps emails in memory and adds assertions.