how to send mail with digital signature using c#

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C#

ASP.NET

Hi,
I am using godaddy mail server to send mail using c#.I am using below code to send mail and it works fine.But code sends email without any digital signature or image embedded on it.Can anyone tell me how to send mail with digital signature and image embedded in the background.What change should i make in below code.Please help.god bless you.

MailMessage mail = new MailMessage();
       SmtpClient SmtpServer = new SmtpClient("relay-hosting.secureserver.net");
       mail.From = new MailAddress("mail@domain.in");
       mail.To.Add("mail@domain.in");
       mail.Subject = "Subject";
       mail.Body = "Body" ;



       SmtpServer.Port = 25;
       SmtpServer.Credentials = new System.Net.NetworkCredential("mail@domain.in", "******");


       SmtpServer.Send(mail);