Include Digital Signature While Sending Email

Hi,
I want to send email in my web application using digital signature. How could i implement the digital signature information in the following code.

SmtpMail.SmtpServer = strSmtpServer;
MailMessage objMail = new MailMessage();
      
strEmailAddress = email@email.com
objMail.From = txtemail.Text.Trim();
objMail.To = strEmailAddress;
objMail.Subject = "Kontakt os";
objMail.Cc = txtemail.Text.ToString().Trim();
objMail.Body = txtcomments.Text.Trim() ;
SmtpMail.Send(objMail);

Could anyone help me out in bringing the solution.

thanks in advance
Senthil