you can use html tag inside string
string body = "You have received a status update from " + Session["name"].ToString() + ", Department : " + Session["deptt"].ToString() + " ,with reference to Assignment ID: " + lblassno.Text + " /Task ID: " + taskid + ". Kindly login TMS on Intranet Portal to view the details.";
body += "<br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><br /><span style="mso-bidi-font-style:normal;font-style: italic;font-size: small;">This is a System generated message,";
body += "kindly do not reply to this.<o:p xmlns:o="#unknown"></o:p></span>";
but ensure when you send mail
MailMessage m = new MailMessage("Omaxe Ltd.<testr@gmail.com>", To);
m.Subject = Subject;
m.Body = Body;
m.IsBodyHtml = true;