zimbra does have good sharing functionality. You can share any e-mail folder you have with another person, group, or domain. Of course this is not a good idea for a central repository because if that person leaves and you delete their account then you've lost the folder.
the better way is to create an account specificly for holding folders like this that the whole company needs access to. so you make like a
company@domain.com account
as far as what you're talking about with automatic e-mail repository, it could probably be done but the logistics of it make it kind of messy. If you ask that all yoru clients communicate only with the address
company@domain.com then you can setup filters for that user to file e-mail according to sender.
But if your clients will be communicating with one or more of your people directly, then filing automaticly could create a lot of duplication not to mention overhead because I think you would have to make postfix send a copy of every incoming e-mail to the
company@domain.com account because last I checked there was no ability to filter to a mounted shared folder (i could be wrong but it hasn't worked for us in 5.0.x)