I have not heard of one. There was a piece of crap one that was included on the Qube 3, but it was slow and very unfeatured. Most people just bolt on SquirrelMail, Open WebMail, etc. and leave it at that.
I'm not sure what you are referring to when you say "separate authentication" as all the ones I have seen use the existing login name and password to validate the user...
I mainly looked for a solution that comes up with the same-styled web frontend of the RaQ and integrates there.
With "separate authentication" I refer to the fact that with a third party webmail solution user needs to log in to the web mail area separately and cannot go from the site admin GUI to the webmail GUI seamlessly.