How does ReflectionTestUtils work in the below code

@Service
public class EmailService {

    private JavaMailSenderImpl mailSender;
    
    @PostConstruct 
    protected void init() {
        mailSender= new JavaMailSenderImpl();
        mailSender.setHost("some.server.com");
        mailSender.setPort("25");
         
        Properties props = mailSender.getJavaMailProperties();
        props.put("mail.transport.protocol", "smtp");
    }
    
    public void sendEmail(String recipient, String subject, String message){   
        SimpleMailMessage simpleMailMessage = new SimpleMailMessage(); 
        simpleMailMessage.setFrom("someone@somewhere.com");
        simpleMailMessage.setTo(recipient); 
        simpleMailMessage.setSubject(subject); 
        simpleMailMessage.setText(message);
        mailSender.send(simpleMailMessage);        
    }
}

Above code is to send emails.

public class EmailServiceTest {
    
    @Rule
    public ExpectedException exception = ExpectedException.none();
    
    @InjectMocks
    private EmailService emailService;
    
    @Mock
    private JavaMailSenderImpl mailSender;
    
    @Before
    public void init() {
        MockitoAnnotations.initMocks(this);  
    }
    
    @Test
      public void testEmail(){
        exception.expect(MailSendException.class);

        ReflectionTestUtils.setField(emailService, "mailSender", mailSender);
        emailService.sendEmail("me.com", "test", "this is a unit test");
        
      }
}

I have written a test case where I expect the mail send to fail, however I am not getting any errors In EmailServicetest testEmail() I expect a MailSendException as the server doesnt exist. Can anyone tell me what might be the reason?