Re: Lack of integrated email/contacts/calendar?
As noted in my original comments Outlook wasn't my choice, it was the users. We tried them with the web client but they didn't like it. They also didn't like the native Zimbra client either. Even if they did though, this also pulls the mail down and caches it locally so would heve been no better in our RDS environment.
Regarding the other posters comment about storing PST files on a network drive, yes they will break as well. It isn't supported and nobody with any sense should try. Since a ZDB file IS a PST file with a different extension it beggars belief that this was Zimbra's suggested fix for the lack of an online mode.
We asked them if there was client on their development roadmap which could work online without having to cache the mailbox. If there was, we may have stuck with it. There wasn't so we went to Exchange.
You may not like it, but the fact is that Outlook is a popular mail client. If you want to make a successful groupware product you need to provide decent support for it.