URLRewrite "Redirect" rule works, but "Rewrite" rule fails (404)

I have following rewrite rule:

<rule name="First Rule" patternSyntax="ECMAScript" stopProcessing="true">
  <match url="^Tracking/(.*)" ignoreCase="true" />
  <action type="Rewrite" url="http://www.example.com/" />
</rule>

This failes, calling http://www.myserver.com/Tracking/123 returns a 404.

However, if I change the action type into Redirect, the rule suddenly works fine, performing a redirect:

<rule name="First Rule" patternSyntax="ECMAScript" stopProcessing="true">
  <match url="^Tracking/(.*)" ignoreCase="true" />
  <action type="Redirect" url="http://www.example.com/" />
</rule>

But I need a rewrite rule, as I need a reverse proxy to an internal server.

I'm working on a Windows 2008R2 Server (IIS 7.5), have "IIS URL Rewrite Module 2" installed,...

What is wrong with the Rewrite rule ?