spring boot and thymeleaf for email only

Question

I have a spring boot application that works correctly.

I need to add a functionality to send emails if certain actions happens.

I have added the thymeleaf code:

@Configuration
public class ThymeleafTemplateConfig {

    @Bean
    public SpringTemplateEngine springTemplateEngine() {
        SpringTemplateEngine templateEngine = new SpringTemplateEngine();
        templateEngine.addTemplateResolver(htmlTemplateResolver());
        return templateEngine;
    }

    @Bean
    public FileTemplateResolver htmlTemplateResolver() {
        FileTemplateResolver emailTemplateResolver = new FileTemplateResolver();
        emailTemplateResolver.setPrefix(EMAILS_TEMPLATES_DIRECTORY);
        //emailTemplateResolver.setSuffix(".html");
        emailTemplateResolver.setTemplateMode("HTML");
        emailTemplateResolver.setCharacterEncoding(StandardCharsets.UTF_8.name());
        return emailTemplateResolver;
    }

}

and below the code for handling the root url /

@Controller
public class IndexController {

    @RequestMapping("/")
    public String index() {
        //return "main.xhtml";
        return "operation.xhtml";
    }

}

When I login to my site I redirect to "http://mysiteip/" and I have set as mapping that the "/ " is the operation.xhtml so at the redirection or if I hit "http://mysiteip/" an error occurs.

 ERROR o.a.c.c.C.[.[.[.[dispatcherServlet] - : - : - Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.thymeleaf.exceptions.TemplateInputException: Error resolving template "operation.xhtml", template might not exist or might not be accessible by any of the configured Template Resolvers] with root cause
org.thymeleaf.exceptions.TemplateInputException: Error resolving template "operation.xhtml", template might not exist or might not be accessible by any of the configured Template Resolvers

Why the thymeleaf tries to render the view? I want to use thymeleaf only for the email part (send emails with html body).