I am trying to send email from a webform. The user is allowed to type a short message in a textbox then send it with a send button. The click event of the send button implements the following block of email-sending code:

		MailMessage loMessage = new MailMessage();
		loMessage.From = this.lblDataFrom.Text;
		loMessage.To = this.lblDataTo.Text;
		loMessage.BodyFormat = MailFormat.Text;
		loMessage.Priority = MailPriority.High;
		loMessage.Body = this.txtMessage.Text;
		SmtpMail.Send(loMessage);

But I get the following error message upon clicking the Send button:

The server response was: 550 5.7.1 Unable to relay for 

BTW, I am runing this from my developing machine which has IIS and SMTP Virtual Server.
Where is the problem?

TIA