What is a good way to check the existence of mail domains on the client side?

Could you tell me how to check the domain on the client side?

I think it would be better to contact DNS to check the existence of the domain of the email address, but I don't think DNS queries can be done on the client side.I think it would be better to consider implementing functions on the server side and calling them from JS on the client side.

Also, checking the existence of a domain in the first place does not confirm the existence of an email address.In short, it is often impossible to make a decision until you send it, so it might be wise to consider measures such as preventing you from registering as a member if you enter the wrong email address.

Since there is no definition of "Do you have a domain?" I have decided to query whois by interpreting it as an intention to verify that it is not a random domain. I will use the whois query service at https://www.whoisxmlapi.com/ in jsonp.See also

varbaseURL='https://www.whoisxmlapi.com/whoisserver/WhoisService?';

varcheckDomain=function(domain, callback){
  var callback=callback||function(){},
      callbackName='__checkDomain_jsonp_callback_'+Date.now();
  window [callbackName] = function(data){
    callback(null,data.WhoisRecord);
  };
  varjsonpElement= document.createElement('script');
  jsonpElement.src=baseURL+'outputFormat=json&domainName='
    + encodeURIC component (domain) + '&callback=' + callbackName;
  jsonpElement.onerror=function(err){
  	callback(err);
  };
  document.body.appendChild(jsonpElement);
};

// example
checkDomain('example.com', function(err,data){
  console.log(err,data);
  if(data.dataError){
  	// If you have this property, you probably can't find whois → Not available
  }
});
checkDomain('example-not-exists-not-available.com', function(err,data){
  console.log(err,data);
  if(data.dataError){
  	// If you have this property, you probably can't find whois → Not available
  }
});

As I noticed from the comments, it might be better to check DNS's MX record.If there is a service available for http requests, I think we can do the same, and as you can see in the comments, you may have a server-side program that accepts http requests yourself.