Async task request

how to create a task request (mail webservice) in order to not wait for response and continue code execution, like:

class HomePageHandler implements RequestHandlerInterface{

    public function handle(ServerRequestInterface $request): ResponseInterface

	    $_post = $request->getParsedBody();

		if( isset($_post['email']) ){
			 * send async request and do not wait it!
  		    $client = new Client();
			$responseMail = $client->send();
			// no need get body
			#$body = $responseMail->getBody();

	    // continue code execution, do not wait response from MAIL API
	    // Example
	    // save user action into DB
		// return message to user, and mail microservice is processing the request 
	    return new JsonResponse(['message'=>'Check your inbox/spam folder'],200);