Failure sending mail: Mailbox unavailable. The server response was: 5.7.1 ... Relaying denied

Many smtp servers use some sort of protection against email spamming.  This only allows trusted users or computers to send email via the server.

1. Open IIS6 Manager using Control Panel --> Administrative Tools.
2. Open SMTP Virtual Server properties.
3. On General tab, Set IP address of the Web server instead of "All Unassigned".
4. In Access tab, click on Relay button, this will open Relay Restrictions dialog.
5. In relay computers list, add the loopback IP address i.e 127.0.0.1 and IP address of the Web server, so that they can pass/relay emails through the SMTP server.