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.