I just talked with Jay Cihla at 01.com (The most experienced provider of Zimbra
hosting, managed & professional services worldwide).
Here is the article he referred me to about the new Zimbra Desktop development:
Zimbra to make HTML5 push in next version | PCWorld
He also mentioned that we should expect it around the release of Zimbra 8.5
(3rd Qtr 2013 or later).
I used Zimbra Desktop everyday, and cannot imagine going back to Outlook, or
trying to use something like Thunderbird, and what else is there?
As far as I understand, this article says the future Zimbra "offline" html5
will be only capable of providing "recent emails" :
"With HTML5, Zimbra will be able to cache the Web client application in local
storage and provide SQL services in Chrome, IE and Firefox for local copies
of recent emails, calendars and contacts."
And in the next I understand : if you want complete access to you mailbox offline,
"use Outlook, Thunderbird or other client"
"For people who need a complete replica of their mailboxes, Zimbra will continue
to provide and expand support for specific desktop client applications from
vendors like Microsoft, Apple and Mozilla."
That's absolutely not good news I think !
I am with an SME company. Some of our users have huge archives of mails, which they store in the file system (i.e. offline, not on the Zimbra server). Storing these archives on the server would require us to increase the (expensive) Zimbra server storage and backup tremendously, keeping the data on networked file system disks is much easer for us. Also, some of the users have several accounts, some through imap on Zimbra, some through pop on other mail servers. For all these reasons, and also for good desktop integration, we have to use a local mail client. In our case, this is Thunderbird.
Not having a mature desktop client that fully exploits Zimbra's capabilities forms a competitive disadvantage for Zimbra, in particular as compared to outlook/exchange. For example, the Thunderbird client does not synchronize addresses (except through the deprecated Zindus Add-on) nor S/MIME certificates with the server.
In my view, not having a decent mail client that integrates well with Zimbra is a major drawback for the Zimbra platform. Now that Mozilla has withdrawn some of its support for Thunderbird, and with Outlook being the only other mainstream mail client out there (at least on the Windows platform), more and more users that need desktop clients will move towards Exchange/Outlook.
I see, however, also a big opportunity for Zimbra here. Now that Thunderbird is in a partially orphaned state, Zimbra could jump in and invest in Thunderbird, either by providing an Add-On for better integration, or by contributing to the Thunderbird application itself. By making Thunderbird a first-class Zimbra client, Zimbra could not only solve its "missing desktop client" problem, but also attract new users to the Zimbra platform.
This is probably just wishful thinking, but I could not stop myself from mentioning it here.