Is there a module for "mail routing" in Drupal 6? Is there a module, with D6 support, that allows one to "route" e-mails.

For example, by default, e-mails would be sent out by the regular mechanism, but for some e-mails, say based on their ID or destination domain, where tracking is interesting, they could instead be passed of to a third party service (such as MailChimp, ExactTarget, MailGun etc).
You could try: https://drupal.org/project/reroute_email
While it's mainly designed to reroute all email to predefined addresses, e.g during development, you could use it as a starting point for similar functionality.