FakeSMTPServerRule.mailbox() doesn't get mailbox content NullPointerExeption

I am facing an issue with testing my email service. When I run test it fails due to Null Pointer Exeption generated on List<EmailModel> receivedMessages = server.mailBox(); But my emails are recieved on fakesmtp server mailbox. Could you direct me how to handle this case?

Test class

@SpringBootTest
@ActiveProfiles("email")
@RunWith(SpringRunner.class)
class EmailServiceTest
{
    @Autowired
    EmailService emailService;

    @Rule
    public FakeSmtpRule server = new FakeSmtpRule(ServerConfiguration.create().port(25).bind("localhost"));

    @Test
    void sendEmail() throws MessagingException, IOException
    {
        // Given
        String to = "test@test.com";
        String title = "Title";
        String content = "Content";
        // When
        emailService.sendEmail(to, title, content);
        // Then
        List<EmailModel> receivedMessages = server.mailBox();

        assertTrue(receivedMessages.size() == 1);

        EmailModel receivedMessage = receivedMessages.get(0);

        assertTrue(receivedMessage.getTo().equals(to));
        assertTrue(receivedMessage.getSubject().equals(title));
        assertTrue(receivedMessage.getEmailStr().equals(content));
    }
}

EmailService class

@Service
public class EmailService
{
    private JavaMailSender javaMailSender;

    @Autowired
    public EmailService(JavaMailSender javaMailSender)
    {
        this.javaMailSender = javaMailSender;
    }
    /*Set email attributes and send it via javaMailSender*/
    public void sendEmail(String receiverEmail, String subject, String message)
    {
        SimpleMailMessage simpleMailMessage = new SimpleMailMessage();
        simpleMailMessage.setTo(receiverEmail);
        simpleMailMessage.setSubject(subject);
        simpleMailMessage.setText(message);
        javaMailSender.send(simpleMailMessage);
    }
}