无法获取函数的ajax回调

我有一个表单供用户注册新帐户。我使用jquery+ajax来检查表单提交时电子邮件地址的可用性。在Jquery代码中，我使用了e.preventDefault（）；以防止在出现任何错误时提交表格。我在电子邮件输入中尝试了现有的电子邮件地址，然后点击提交表格。它允许提交表格。它不应该这样做，因为ajax reponseText返回true意味着电子邮件地址已经存在于数据库中。

有人能告诉我如何修复我的代码吗？这样，如果ajax响应返回true，它将阻止表单提交并显示错误。

我试着阅读和阅读这篇文章，但尝试了很多次都失败了。

这是我的表格：

<form role="form" method="post" id="signupForm" action="index.php?view=signup-gv">
                <div class="col-xs-6 border-right">
                    <div class="form-group">
                        <label for="exampleInputEmail1">Full Name</label>
                        <input type="text" class="form-control" id="regname" name="regname" placeholder="Full Name">
                    </div>
                    <div class="form-group">
                        <label for="exampleInputEmail1">Email Address</label><span id="emailcheck"></span>
                        <input type="email" class="form-control" id="regemail" name="regemail" placeholder="Enter email">
                    </div>
                </div>
                <div class="form-group col-xs-6">
                    <label for="exampleInputPassword1">Password</label>
                    <input type="password" class="form-control" id="regpass" name="regpass" placeholder="Password">
                </div>
                <button style="position:relative; left: 15px; top: 10px;" class="btn btn-default" name="register" id="register">Register</button>
            </form>

这里是我的jquery代码：

$(document).ready(function(){
$('#regname').focus(); 
$('#signupForm').submit(function(e) {
var regname = $('#regname'); 
var regemail = $('#regemail'); 
var regpass = $('#regpass'); 
var register_result = $('#register_result'); 
register_result.html('Loading..'); 
if(regname.val() == ''){ 
    regname.focus();
    register_result.html('<span class="errorss"> * Full name can not be blank</span>');
    e.preventDefault();
}
else if ($.trim(regemail.val()).length == 0) {
    regemail.focus(); 
    register_result.html('<span class="errorss">* Email address can not be blank</span>');
    e.preventDefault();
}
else if(regpass.val() == ''){ 
    regpass.focus();
    register_result.html('<span class="errorss">* Password can not be blank</span>');
    e.preventDefault();
}
emailCheck().done(function(r){
    if(r){
        $('#regemail').focus(); 
        $('#register_result').html('<span class="errorss"> This email address is already existed. Please choose another one </span>');
        e.preventDefault();
        }
    }); 
});
});
function emailCheck() {
var regemail = $('#regemail'); 
var emailcheck = $('#emailcheck');
emailcheck.html(''); 
var UrlToPass = {regemail:regemail.val()} ;
    $.ajax({ 
    type : 'POST',
    cache: false,
    data : UrlToPass,
    url  : 'emailcheck.php',
    success: function(responseText){ 
        if(responseText == 0){
            return false; // good to go
        }
        else{
            emailcheck.html('<span class="errorss"> This email is existed.</span>');
            return true; // This email is registered. Please try different one
        }
    }
    });

}