<script type="text/javascript">
        var form;
        Ext.onReady(function () {
            form = new Ext.form.FormPanel({
                baseCls: 'x-plain',
                layout: 'absolute',
                //url: 'http://localhost:1683/Handler1.ashx',
                defaultType: 'textfield',

                items: [{
                    x: 0,
                    y: 5,
                    xtype: 'label',
                    text: '收件人:'
                }, {
                    x: 60,
                    y: 0,
                    name: 'to',
                    anchor: '100%'  // anchor width by percentage
                }, {
                    x: 0,
                    y: 35,
                    xtype: 'label',
                    text: '主题:'
                }, {
                    x: 60,
                    y: 30,
                    name: 'subject',
                    anchor: '100%'  // anchor width by percentage
                }, {
                    x: 0,
                    y: 60,
                    xtype: 'textarea',
                    name: 'msg',
                    anchor: '100% 100%'  // anchor width and height
                }]
            });

            var window = new Ext.Window({
                title: '发送邮件',
                width: 500,
                height: 300,
                minWidth: 300,
                minHeight: 200,
                layout: 'fit',
                plain: true,
                bodyStyle: 'padding:5px;',
                buttonAlign: 'center',
                items: form,

                buttons: [{
                    text: '发送', handler: submit
                }, {
                    text: '取消'
                }]
            });

            window.show();
        });


        function submit() {
            if (!form.getForm().isValid()) return;
            form.getForm().submit({
                waitMsg: '正在提交数据',
                waitTitle: '提示',
                //url: '../../Handler1.ashx',
                url: 'Home/GGG',
                method: 'GET',
                success: function (form, action) {
                    Ext.Msg.alert('提示', '保存成功啊啊');
                },
                failure: function (form, action) {
                    Ext.Msg.alert('提示', '原因如下：' + action.result.errors.info);
                }
            });
        }

  </script>

public ActionResult GGG()
        {
            //Response.Write("{ success: false, errors:{info: '错误了'} }");
            Response.Write("{success:true}");
            Response.End();
            return View();
        }

那就是你访问的页面出现异常了，看看是哪里出现错误了。看看这个请求执行了那个页面的代码