What is wrong with this Spring Boot class using field injection?

What is wrong with this Spring Boot class using field injection?

@Component
public class EmailService {
  @Autowired
  private SmtpClient smtpClient;

  public void sendEmail() {
    smtpClient.connect();
  }

  public EmailService(SmtpClient smtpClient) {
    this.smtpClient = smtpClient;
  }
}