I think thats happening because you are trying to send an email via SmtpClient class. It is trying to look up the from address for this MailMessage from the property or config section and could not able to find it there. You can specify the From address when you call Send() or with the From property of MailMessage or in the config file. The System.Net config section is defined at
MSDN[
^]. You can also check this
link[
^] as well.
Good luck,
OI