This is a new module that is of interest to developers, Reroute email.
What it does is that it intercepts all emails send from a Drupal site and reroutes them to an email address that the site admin defines.
This is useful for use cases where a copy of a live site is used for testing, and we don't want the email from the test site to reach the users of the real site, since it would be like spamming. It is also useful when you, the developer or tester, want to check various email functions from a site without having to check a dozen email addresses.
This module is a demonstration on how to use hook_mail_alter.
Written for Drupal 5.x or later.