Here's an interesting approach - until it's changed in
Bug 22819 - reply to another user's message is always on-behalf-of is changed (probably will give a drop down choice of how to send it) you can take advantage of this behavior without the need for persona's - assuming all involved are using ZWC:
The test:
Create 5 accounts.
1) Shared
2) User1 -manager of shared's inbox & sent
3) User2 -viewer of shared's inbox & sent
4) Test1
5) Test2
Send message from user1 to test1 (or user2 to test2) and both user1 & user2 can see the sent mail - best if you're using conversation view so it's shown when not in the shared sent folder and working in shared inbox.
Zimbra will show this as 'sending on behalf'
The headers look like:
From:
shared@domain.com (so to test1 or test2's perspective it's shared sending the email)
Sender:
user1@domain.com (so yes test1 or test2 can technically view the mail headers and see which person sent it if not using ZWC - some mail clients may display this)
To:
test1@domain.com