Windows XP IIS SMTP Relay Setup

Install IIS from Windows component installer

Control Panel -> Administrative Tools -> Internet Information Services

(local computer) -> Default SMTP Virtual Server -> Domains

Right Click on Domains -> New -> Domain...

Select Remote, Next

Enter domain name (eg, google.com), Next

Right click on new domain name -> Properties

Check 'Allow incoming mail to be relayed to this domain'

Route Domain: Select 'Use DNS to route to this domain'