i thing you want to add drop down value while send mail to end user Am Right?
if am right just call the drop down id where you want to add and send the value to mail.thats it.
hope this will help you.
MyMailL.IsBodyHtml = true;
If you make that property true, you can send any html code as body of the email. So Now you can write the code for drop down, text box whatever you want. Then send the email. So the user can use receive the email as html.
Direction is important than speed
MailMessage mailMsg = new MailMessage();
mailMsg.IsBodyHtml = true;
mailMsg.Body = "<input type=\"text\" name=\"txt1\" value=\"Type Here!\" /><BR/><select name=\"ddl1\"><option value=\"Item1\">Hi 1</option></select>";