class required a single bean, but 2 were found:

You may simply name your bean using

@Bean(name="mockMailSender")
public MockMailSender getMockMailSender() { }

and then decorate the autowired field with

@Autowired
@Qualifier("mockMailSender")
private MailSender smtpkMailSender;

Since you're using java config you should mark config method with @Primary annotation, and not a class:

@Configuration
public class MailConfig {
    @Bean
    public SmtpkMailSender getSmtpkMailSender(){
        return new SmtpkMailSender();
    }

    @Bean
    @Primary
    public MockMailSender getMockMailSender(){
        return new MockMailSender();
    }
}

You can use @Qualifier annotation for specify which specific type of your implementation, you want to autowire.

@RestController
public class MailController {

   @Autowired
   @Qualifier("smtpkMailSender")
   private MailSender smtpkMailSender;

   @RequestMapping("/send")
   public String send(){
      smtpkMailSender.sender("Person", "Important", "Take Care");
      return "mail is sent";
  }

}