JSFiddle - React, Tailwind, and code Playground

HTML

<form action="mail.php" method="post" id="my_form">

                <input type="text" name="name" size="25" title="Введите ваше имя" value="Ваше имя" id="name">
                <input type="text" name="email" size="25" value="Ваш email" id="email">
                <input type="image" src="images/button.png" class="button" disabled="disabled" >
            </form>

JavaScript

var regV = /[-\w.]+@([A-z0-9][-A-z0-9]+\.)+[A-z]{2,4}/;
$('#email').focus(function(){
    $(this).addClass('onFocus');
    if($(this).val() == $(this).attr('value')){
        $(this).val('');
    }
    })
    .blur(function(){
        var userEmail = $(this).val();
        $(this).removeClass('onFocus');
        if($(this).val() == ''){
            $(this).val($(this).attr('value'));
        }

      else if(userEmail.search(regV) == -1){
            $('#button').attr('disabled', 'disabled')
            $('.info').prepend('<p>Введите коректный Email</p>').slideDown(1000);
        }

    });