Edit in JSFiddle

var btn = document.getElementById('btn');
 
btn.addEventListener('click', function (e) {
    var emailField = document.getElementById('email'),
        emailRegex = /^[-a-z0-9~!$%^&*_=+}{\'?]+(\.[-a-z0-9~!$%^&*_=+}{\'?]+)*@([a-z0-9_][-a-z0-9_]*(\.[-a-z0-9_]+)*\.(aero|arpa|biz|com|coop|edu|gov|info|int|mil|museum|name|net|org|pro|travel|mobi|[a-z][a-z])|([0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}))(:[0-9]{1,5})?$/,
        errors = document.getElementById('errors'),
        errorContainer = document.createElement('li'),
        errorMessage = document.createTextNode('Wprowadzono niepoprawny adres email!');
    
    errors.innerHTML = '';
    if (emailRegex.test(emailField.value)) {
        return true;
    }
    
    errorContainer.appendChild(errorMessage);
    errors.appendChild(errorContainer);
    emailField.focus();
    emailField.select();
    e.preventDefault();
    return false;
});
<ul id="errors"></ul>

<form action="#" method="POST">
    <label for="email">Adres email:</label>
    <input id="email" type="text" name="email" />
    <input id="btn" type="submit" value="Wyƛlij" />
</form>