We had a call from a user which confused us for a while, too. Basically it boils down to this:
1) A zimbra user receives an email from "John Smith" <email@example.com>
2) Recipient has entry in his zimbra contact list, "Joanne Smith" <firstname.lastname@example.org>
3) Zimbra shows the message as from "Joanne Smith", even though the From: header in the message says "John Smith"
I'm not sure if this is a bug or a feature, but we would like to be able to disable or fix it. Any ideas?
Edit: I just found that, in the Zimbra web interface, disabling the "email" Zimlet prevents this undesirable behavior. However, our user who complained had experienced the issue on his Blackberry, which is not affected by zimlets.