[QUOTE]This feature requires you to use a Microsoft Exchange Server 2000 or later e-mail account. The recipient of the mail you want to recall must also be using an Exchange server e-mail account.[/QUOTE]Looks like this is an Exchange feature; I wouldn't expect it to work if you have Zimbra as your back-end.
I assume jholder's comment was just giving an example of how the feature does get real-world use.