I recently switched our company over to using the XMPP server built into Zimbra and so far it has worked excellently. There is one feature that I really would like to see however: new mail notifications. I have been unable to find any reference to this at all and as far as I can tell it doesn't currently work. For example if you connect to the Gmail XMPP server (and your IM client supports it) you can receive notifications when you receive a new email. I use Pidgin myself but I am sure that other IM programs support this feature.
Is this currently supported at all for the Zimbra server or should I submit an enhancement request?