I have configured all that you say, but that error still remain .
I have tried send mail with my follow code :
using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Web.Mail;

public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
System.Web.Mail.MailMessage objEmail = new MailMessage();
objEmail.To = "phan@office.awsystems.biz";
objEmail.From = "phan@office.awsystems.biz";
objEmail.Subject = "Test Email";
objEmail.Body = "Test 1 phat thoi , chu co gi dau ????";
objEmail.Priority = MailPriority.High;
SmtpMail.SmtpServer = "myserver";
try
{
SmtpMail.Send(objEmail);
Response.Write("Your Email has been sent sucessfully - Thank You");
}
catch (Exception exc)
{
Response.Write("Send failure: " + exc.ToString());
}
}
}

it run well . But with follow key in kentico:
<add key="CMSSMTPServer" value="myserver" />
<add key="CMSSendWorkflowEmailsFrom" value=" phan@office.awsystems.biz" />
it doesn't . Did I do not correctly ???