what is the difference between these two methods?

The difference is one SendMailAsync uses the new async/await technology and the other uses the old callback technology. And more importantly, the Object that's passed is simply passed into the event handler as the userState when the method completes.

Firstly, they both work asynchronously.

However, SendAsync has existed since .NET 2. In order to maintain backwards compatiblity whilst supporting the new Tasks system SendMailAsync was added.

SendMailAsync returns a Task rather than void and allows the SmtpClient to support the new async and await functionality if required.

  //SendAsyncpublic class MailHelper{    public void SendMail(string mailfrom, string mailto, string body, string subject)    {        MailMessage MyMail = new MailMessage();        MyMail.From = new MailAddress(mailfrom);        MyMail.To.Add(mailto);        MyMail.Subject = subject;        MyMail.IsBodyHtml = true;        MyMail.Body = body;        MyMail.Priority = MailPriority.Normal;        SmtpClient smtpMailObj = new SmtpClient();        /*Setting*/        object userState = MyMail;        smtpMailObj.SendCompleted += new SendCompletedEventHandler(SmtpClient_OnCompleted);        try        {            smtpMailObj.SendAsync(MyMail, userState);        }        catch (Exception ex) { /* exception handling code here */ }    }    public static void SmtpClient_OnCompleted(object sender, AsyncCompletedEventArgs e)    {        //Get the Original MailMessage object        MailMessage mail = (MailMessage)e.UserState;        //write out the subject        string subject = mail.Subject;        if (e.Cancelled)        {            Console.WriteLine("Send canceled for mail with subject [{0}].", subject);        }        if (e.Error != null)        {            Console.WriteLine("Error {1} occurred when sending mail [{0}] ", subject, e.Error.ToString());        }        else        {            Console.WriteLine("Message [{0}] sent.", subject);        }    }    //}//SendMailAsyncpublic class MailHelper{    //    public async Task<bool> SendMailAsync(string mailfrom, string mailto, string body, string subject)    {        MailMessage MyMail = new MailMessage();        MyMail.From = new MailAddress(mailfrom);        MyMail.To.Add(mailto);        MyMail.Subject = subject;        MyMail.IsBodyHtml = true;        MyMail.Body = body;        MyMail.Priority = MailPriority.Normal;        using (SmtpClient smtpMailObj = new SmtpClient())        {            /*Setting*/            try            {                await smtpMailObj.SendMailAsync(MyMail);                return true;            }            catch (Exception ex) { /* exception handling code here */ return false; }        }    }}