Anonymous Email like Craigslist

Use the PHP mail function –

$to = '[email protected]';
$subject = 'Your subject';
$message = 'Your message';
$hearers = 'FROM: [email protected]';
mail($to, $subject, $message, $headers);

Sorry I wasn’t clear originally…

I want the registered user who created the post to receive the email but I don’t want the visitor sending the email to see the registered users email address.

So if you have that in your code, the sender will not see any of those details. The email will be generated and sent on page load (or whenever you call that function).