Spring boot @PostMapping for login

From what you wrote, I assume that you want to send email only when user performs a successful login. You could write your implementation of AuthenticationSuccessHandler to send an email when user logs in succesfully.

import java.io.IOException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.springframework.security.core.Authentication;
import org.springframework.security.web.authentication.AuthenticationSuccessHandler;
import org.springframework.stereotype.Component;
        
@Component
public class MyNotifyingAuthSuccessHandler implements AuthenticationSuccessHandler {
        
  public void onAuthenticationSuccess(HttpServletRequest request,
HttpServletResponse response, Authentication authentication) throws IOException {
        
       //send email (do it asynchronously if you can)
        
       //redirect to your homepage (page where user should land after login)
    }
}

Sending an email sometimes takes a lot of time, so consider doing that async not to block the login operation for too long, if your business requirements allow you to do that.