This is probably a relay error.
Make sure you can relay through the SmtpMail.SmtpServer either by your IP address,
by your MailMessage.From address, or if you need to authenticate, check out
3.8 How do I authenticate to send an email?
If SmtpMail.SmtpServer is set to "127.0.0.1" or "localhost", and you are using the built in IIS SMTP Service, you can allow relaying for 127.0.0.1 by
- Opening the IIS Admin MMC
Right-Clicking on the SMTP Virtual Server and selecting Properties
On the Access tab, click the Relay BUTTON
Grant 127.0.0.1 (or the IP address used by System.Web.Mail) to the Computers list.
Close all dialogs
Restarting the SMTP Service
Also, be sure to check the InnerException for any additional, helpful information.