I'm trying to send and Email to somebody, but I faced to problem. Actually I'm working with C# .Net and my code body is here as follows: using System.Net; using System.Net.Mail; SmtpClient SMTPClientObj = new SmtpClient(); SMTPClientObj.UseDefaultCredentials = false; SMTPClientObj.Credentials = new NetworkCredential(" admin@Domain.com", "Admin password"); SMTPClientObj.Host = "Mail.inno-tech.com"; SMTPClientObj.Port = 587; SMTPClientObj.EnableSsl = true; SMTPClientObj.Send(" admin@Domain.com", " somebodyEmail@yahoo.com", "test", "this is test") I faced to "Server does not support secure connections" problem. when I used this code SmtpClient smtpClient = new SmtpClient(); NetworkCredential basicCredential =new NetworkCredential("admin", "admin Password", "Mail.inno - tech.com"); MailMessage message = new MailMessage(); MailAddress fromAddress = new MailAddress(" sam@yahoo.com"); smtpClient.Host = "Mail.inno-tech.com"; smtpClient.UseDefaultCredentials = false; smtpClient.Credentials = basicCredential; message.From = fromAddress; message.Subject = "your subject"; message.IsBodyHtml = true; message.Body = "

your message body

"; message.To.Add(" David@gmail.com"); try { smtpClient.Send(message); } catch (Exception ex) { MessageBox.Show(ex.Message); } I faced to this problem "Mailbox unavailable. The server response was: No such user here" . It seems that David@gmail.com in not valid for Host address(Mail.inno - tech.com). As a matter of fact, my problem is I can not send an Email from all mail boxes to all any other mail boxes. I just can send an Email from all mail boxes to Email addresses which are valid for my host address(Mail.inno - tech.com). I wonder whether I should set some setting as a configuration setting in which I define for Host to accept all mail addresses as a receiver or there is other solution to solve it. In advance I really appreciate you for your help.