Using shared mailboxes with the ZCO results in different behaviour when
compared with the ZCS web interface:
1. user2 shares their mailbox with user1
2. when user1 uses the web interface to reply to an e-mail that's in user2's inbox, the message has the From: header set to "user2@", as you would expect.
3. however, when user1 uses the ZCO (Office 2010 and Office 2003), and replies to a message in user2's inbox, the From: header is set correctly to "user2@", but the message also has a "Reply-To: user1@" header.
Using personas can sort-of work around this, but it's tedious, and does not happen automatically in ZCO.
Please alter the ZCO to mirror the behaviour of the web interface when replying to mail when using shared mailboxes.