Trying to identify the open handle in an email sending module with nodemailer

Question

I am creating a node module to send emails with nodemailer. The implementation is similar to what you see in the nodemailer module demo:

'use strict';

import nodemailer, { Transporter } from 'nodemailer';
import { EmailSender, EmailReceiver } from '../types/email';

declare global {
  namespace NodeJS {
    interface ProcessEnv {
      SMTP_PORT: number;
      SMTP_HOST: string;
      SMTP_USER: string;
      SMTP_PASSWORD: string;
    }
  }
}

async function sendEmail (sender: EmailSender, receiver: EmailReceiver, subject: string, text: string, html: string) {
  const transporter: Transporter = nodemailer.createTransport({
    host: process.env.SMTP_HOST ?? '',
    port: process.env.SMTP_PORT,
    secure: process.env.NODE_ENV !== 'development',
    auth: {
      user: process.env.SMTP_USER,
      pass: process.env.SMTP_PASSWORD,
    },
  });

  try {
    const info = await transporter.sendMail({
      from: `"${sender.name}" <${sender.email}>`,
      to: `"${receiver.name}" <${receiver.email}>`,
      subject: subject,
      text, html
    });

    return info.messageId;
  } catch (error) {
    console.error(error);
  }

}

export { sendEmail };

After sending the email, the email id is received. Apparently it works fine, but when implementing the test with jest an open handle is detected.

import { createTestAccount, TestAccount } from 'nodemailer';
import { sendEmail } from '../src/email';
import { EmailSender, EmailReceiver } from '../src/types/email';

let sender: EmailSender;
let receiver: EmailReceiver;
let testAccount: TestAccount | null;

beforeAll(async () => {
  testAccount = await createTestAccount();
  process.env.NODE_ENV = 'development';
  process.env.SMTP_USER = testAccount.user;
  process.env.SMTP_PASSWORD = testAccount.pass;
  process.env.SMTP_HOST = 'smtp.ethereal.email';
  process.env.SMTP_PORT = 465;

  sender = {
    name: 'Test Kemus',
    email: '[email protected]'
  };

  receiver = {
    name: 'Test Receiver',
    email: '[email protected]'
  };
});

afterAll(() => {
  testAccount = null;
});

describe('Send email function', () => {
  it('should be defined and truthy', () => {
    expect(sendEmail).toBeDefined();
    expect(sendEmail).toBeTruthy();
  });

  it('should be a function', () => {
    expect(sendEmail).toBeInstanceOf(Function);
  });
});

describe('Send email', () => {
  test('should send an email', async () => {
    const subject = 'Test subject';
    const text = 'Test text';
    const html = '<h1>Test html</h1>';

    try {
      const info = await sendEmail(sender, receiver, subject, text, html);

      expect(info).toBeTruthy();
      expect(info).toBeInstanceOf(String);
    } catch (error) {
      console.error(error);
    }
  });
});

The test that gives an error is "should send an email":

thrown: "Exceeded timeout of 5000 ms for a test. Use jest.setTimeout(newTimeout) to increase the timeout value, if this is a long-running test."

When I run the tests with the --detectOpenHandles flag, it shows me the following error:

Jest has detected the following 1 open handle potentially keeping Jest from exiting:

  ●  TCPWRAP

      27 |
      28 |   try {
    > 29 |     const info = await transporter.sendMail({
         |                                    ^
      30 |       from: `"${sender.name}" <${sender.email}>`,
      31 |       to: `"${receiver.name}" <${receiver.email}>`,
      32 |       subject: subject,

      at SMTPTransport.getSocket (node_modules/nodemailer/lib/smtp-transport/index.js:70:16)
      at SMTPTransport.send (node_modules/nodemailer/lib/smtp-transport/index.js:136:14)
      at node_modules/nodemailer/lib/mailer/index.js:218:34
      at Mail._processPlugins (node_modules/nodemailer/lib/mailer/index.js:266:20)
      at node_modules/nodemailer/lib/mailer/index.js:188:18
      at processPlugins (node_modules/nodemailer/lib/mailer/index.js:279:28)
      at node_modules/nodemailer/lib/mailer/index.js:287:17
      at Mail._convertDataImages (node_modules/nodemailer/lib/mailer/index.js:391:20)
      at Mail._defaultPlugins.compile (node_modules/nodemailer/lib/mailer/index.js:31:41)
      at processPlugins (node_modules/nodemailer/lib/mailer/index.js:283:13)
      at Mail._processPlugins (node_modules/nodemailer/lib/mailer/index.js:291:9)
      at Mail.sendMail (node_modules/nodemailer/lib/mailer/index.js:168:14)
      at src/email/index.ts:29:36
      at src/email/index.ts:8:71
      at Object.<anonymous>.__awaiter (src/email/index.ts:4:12)
      at sendEmail (src/email/index.ts:19:12)
      at tests/send-email.test.ts:50:35
      at tests/send-email.test.ts:8:71
      at Object.<anonymous>.__awaiter (tests/send-email.test.ts:4:12)
      at Object.<anonymous> (tests/send-email.test.ts:44:43)

I can't understand what the open handle is.