This single-instance storage is going to encourage bad habits! I just found myself preparing an e-mail that needs to be sent to multiple recipients - including several within my own organization - yet it's not necessary for everyone (in-house) to get the large attachment. As I was getting ready to split the message into two parts for distribution I remembered - I can just send it to EVERYBODY in-house and it only occupies a single instance - which it would anyway just being in my "sent" folder! Blasting away...
You're quite the enabler, Timo! Now any mail admin who tries to lecture users on only sending attachments to those in-house personnel who really need it, because otherwise it's wasteful of precious server storage space, is going to hear the response, "Dude - don't stress out so much - just setup Dovecot with SIS!".
Thanks again!
Daniel