Email 'mapping' to localhost for local email solution for valid email address

I'm on Windows using XAMPP and have Mercury Mail set up working fine for local mail. To get this working I need to use addresses such as, user@localhost

But it's a pain when I use email address validation in my scripts that require a properly formed email address, ie, with a domain extension like .com, .net, .co.uk etc on the end.

If I set up an address like [email protected], or even [email protected], is there a place (ini, conf, hosts file or similar) where I can get this to 'map' to user@localhost?