You could do this by configuring all user email accounts to 'deliver locally and forward' email to the appropriate manager, then configure rules in the manager's email client (or using the Mailsorting contrib) to sort the email into folders based on the original 'To:' field.
That sounds interesting... but just for received e-mail... outgoing e-mail wouldn't be stored that way.
Or, install UserManager and Vacation, and have all users configure a vacation message that says 'I'm out. Talk to my manager'. (The admin could manually create vacation messages for users who forgot to do so).
That would be a laborious job... not an option right now... maybe as last option.
Zarafa provides email folder sharing like Groupwise and Exchange, but looks a bit 'bleeding edge' for a production server (and has some odd licensing in the free/community edition - 2 full 'exchange' users, unlimited imap/webmail/activesync users).
They all uses Thunderbird as client... so Outlook licensing is not a problem... I'd love to test Zarafa... I must to be prepared to have problems with mobile phone (sync) ... not sure if it's something I wanna to do ... but it's a REALLY temptation

Zarafa has lots of ENTERPRISE options, proxy e-mail, sync, ... all included.
I tried to figure out how to setup shared email folders using symlinks and permissions and dovecot-shared.
let us to know if you have good results.
For now I'll try to demove my client of this request.... or find the less worst way to do the job.