Mailbox unavailable. The server response was: 5.7.1 unable to relay for info@site.com

Use the following configuration in Web. config:
<System.net>
<Mailsettings>
<SMTP from = "info@site.com">
<Network host = "localhost" Port = "25"/>
</SMTP>
</Mailsettings>
</System.net>

Use localhost to send emails directly.
Deployment on the server encountered an exception: Mailbox unavailable. The server response was: 5.7.1 unable to relay for info@site.com

SMTP server for which IIS6 manager needs to be configured
1. Open IIS6 Manager using Control Panel => Administrative Tools.
2. open SMTP virtual server properties.
3. On the 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 IP addres 127.0.0.1.