* In order to reduce SPAM, all email addresses will be obfuscated so they cannot be read by spam bots.
For example: 'test@test.com' will become 'test (at) test dot com'.

Not a gateway or local host.
This is 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