Your usage of the word 'shared' is misleading. I assume you mean your just getting the mail delivered to multiple users, not working from a common folder.
Shared email folders (share an inbox like you would a calendar) is coming in v5.0.
Following the mail delivery to multiple accounts meaning for 'shared':
Sound's like you need to get them to understand the difference between
Alias Addresses, Canonical Addresses, & Mail Identities:
Alias Addresses > Allows inbound mail for
first.lastname@domain.com to go to the user
name@domain.com (currently in admin console/shell access only -though there a request open for user made aliases)
Canonical Addresses >displays in the 'From:" field of messages that are sent. So if you want to appear as
first.last@domain.com instead of
name@domain.com on emails that you send. (again user cannot set this-admin console/shell access only)
Mail Identities > The user can select when they want to appear as
first.last@domain.com or
first.m.last@domain.com or just
last@domain.com
Most people use
Mail Identities to appear like their signed in to another service-send a message to personal friends without leaving your work account etc. So from zimbra you could send mail as
user@gmail.com without switching clients/browsers/etc. However that would only take care of sending, this does not work for receiving unless there is an inbound alias setup.
Domain wide:
ManagingDomains - ZimbraWiki