There are now a couple of choices for this issue in this thread.
I am doing exactly that (it appears) with Ultimate Spool Manager; I set up an archive email account for one user - in this case a separate SM user, which gives them all their incoming and outgoing emails. Works fine. No reason this could not be set up for more users as desired. The user could, intentionally or not, of course, delete emails from this account, but they remain in the SM message archive anyway.
Good to hear that your Barracuda solution is working, so this reply is for others who may have a similar need.
It has been mentioned elsewhere in this forum that the "Declude for SmarterMail" product will also do this filtering, even if you do not activate Declude's anti-spam filtering:
I have not tried it - YMMV.