How do I test the JavaMailSender of Spring

To add a more recent answer to this question and as the linked blog post from 2012 seems to be down occasionally, here's a full example of using GreenMail for writing integration tests with JUnit 5 (assuming you're using Spring Boot's auto-configuration for the JavaMailSender).

First, make sure to override the credentials and location of the mail server. You can add an application.yml file inside src/test/resources for this purpose:

spring:
  mail:
    password: springboot
    username: duke
    host: 127.0.0.1
    port: 3025 # default protocol port + 3000 as offset
    protocol: smtp
    test-connection: true

Next, Spring Boot's auto-configuration will configure the JavaMailSender to connect to GreenMail for your tests. You can use GreenMail's JUnit Jupiter extension to conveniently start/stop a GreenMail server for your tests:

<dependency>
  <groupId>com.icegreen</groupId>
  <artifactId>greenmail-junit5</artifactId>
  <version>1.6.1</version>
  <scope>test</scope>
</dependency>

... resulting in the following sample test:

@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
class JavaMailSenderIT {

  @RegisterExtension
  static GreenMailExtension greenMail = new GreenMailExtension(ServerSetupTest.SMTP)
    .withConfiguration(GreenMailConfiguration.aConfig().withUser("duke", "springboot"))
    .withPerMethodLifecycle(false);

  @Autowired
  private JavaMailSender javaMailSender;

  @Test
  void shouldUseGreenMail() throws Exception {

    SimpleMailMessage mail = new SimpleMailMessage();
    mail.setFrom("[email protected]");
    mail.setSubject("A new message for you");
    mail.setText("Hello GreenMail!");
    mail.setTo("[email protected]");


    javaMailSender.send(mail);

    // awaitility
    await().atMost(2, SECONDS).untilAsserted(() -> {
      MimeMessage[] receivedMessages = greenMail.getReceivedMessages();
      assertEquals(1, receivedMessages.length);

      MimeMessage receivedMessage = receivedMessages[0];
      assertEquals("Hello GreenMail!", GreenMailUtil.getBody(receivedMessage));
      assertEquals(1, receivedMessage.getAllRecipients().length);
      assertEquals("[email protected]", receivedMessage.getAllRecipients()[0].toString());
    });
  }
}

You can also use Testcontainers to start a local GreenMail container as your local sandbox email server.

If your goal is to use just Junit/Mockito and test how MimeMessage was formed before sending then the configuration below should be sufficient:

public class EmailServiceTest {

    private EmailServiceImpl emailServiceImpl;

    private JavaMailSender javaMailSender;

    private MimeMessage mimeMessage;

    @Before
    public void before() {
        mimeMessage = new MimeMessage((Session)null);
        javaMailSender = mock(JavaMailSender.class);
        when(javaMailSender.createMimeMessage()).thenReturn(mimeMessage);
        emailServiceImpl = new EmailService(javaMailSender);
    }

    @Test
    public void emailTest() {
        String recipient = "[email protected]"
        EmailRequest request = new EmailRequest();
        request.setRecipient(recipient);
        emailServiceImpl.send(request);
        assertEquals(recipient, mimeMessage.getRecipients(RecipientType.TO)[0].toString());
    }
}

You can use a test SMTP server, like Dumbster. See the example below:

@Test
    public void sendSimpleEmailWithCC() {
        // Runs a Dumbster simple SMTP server - default config
        SimpleSmtpServer server = SimpleSmtpServer.start();
        String from = "[email protected]";
        String to = "[email protected]";
        String messageText = "Good message";
        String title = "Test message";
        String cc = "[email protected]";
        Assert.assertTrue(mailSender
                .sendEmail(from, to, cc, title, messageText));
        server.stop();
        Assert.assertTrue(server.getReceivedEmailSize() == 1);
        Iterator emailIter = server.getReceivedEmail();
        SmtpMessage email = (SmtpMessage) emailIter.next();
        Assert.assertTrue(email.getHeaderValue("From").equals(from));
        Assert.assertTrue(email.getHeaderValue("To").equals(to));
        Assert.assertTrue(email.getHeaderValue("Cc").equals(cc));
        Assert.assertTrue(email.getHeaderValue("Subject")
                .equals("Test message"));
        Assert.assertTrue(email.getBody().equals(messageText));
    }

I've done it using GreenMail. Take a look at my blog post about it where you'll also find a working example.