Introduction:
In this i am going to explain how to send a mail more than one client asynchronously using Asp.Net/c#.

Main:
Normally we using smtp.Send for sending smtp mails,For asynchronous mail communication we have a new option
in asp.net2.0 called sendasync.

MailMessage NetProgrammingHelp_Mail = new MailMessage(); NetProgrammingHelp_Mail.To.Add("netprogramminghelp@gmail.com"); NetProgrammingHelp_Mail.To.Add("silverlightscripting@gmail.com"); NetProgrammingHelp_Mail.From = new MailAddress("sample@gmail.com"); NetProgrammingHelp_Mail.Subject = "Hi,This is my mail with attachment"; NetProgrammingHelp_Mail.Body = "Enjoy this Code"; NetProgrammingHelp_Mail.IsBodyHtml = true; NetProgrammingHelp_Mail.Attachments.Add(new Attachment(@"C:\sample.txt", sample.txt)); SmtpClient smtp = new SmtpClient(); object local_status = mail; smtp.Host = "SmtpNetProgrammingHelp.com"; smtp.Credentials = new System.Net.NetworkCredential ("NetProgrammingHelp_MailID@gNetProgrammingHelp_Mail.com", "NetProgrammingHelp_MailPassword"); //Use yours own userid and password,for ex,gmail userid and password. smtp.EnableSsl = true; smtp.SendCompleted += new SendCompletedEventHandler(smtp_SendCompleted); smtp.SendAsync(mail, local_status);     public void smtp_SendCompleted(object sender, System.ComponentModel.AsyncCompletedEventArgs e) { MailMessage mail = e.UserState as MailMessage; if (!e.Cancelled && e.Error!=null) { Response.Write("Mail sent successfully"); } }

Conclusion:
Hope this helps,
Happy Coding.