I downloaded Zimbra Desktop RC1 to see how it's progressed.
While it does seem to have potential, there are some usability issues, in particular with Gmail accounts.
As a side note: I don't get why Yahoo only offers IMAP using Zimbra. I could accept that if this were for free mail users, but they should at least be offering unfettered IMAP access to paid users. I moved my main mail accounts FROM Yahoo to Gmail precisely because of this...
In any case, I like that there's built in support for Gmail accounts but when you use it I can't find any way to enable syncing to all Gmail IMAP folders, INCLUDING All Mail, Starred, etc. I did read somewhere that this is by design to avoid downloading duplicate messages. However, Gmail pushes the idea of keeping all mail archived rather than deleting mail. If you remove all labels from a message/conversation in Gmail, it will ONLY be accessible through the All Mail label/folder. Since Zimbra does not sync to All Mail, it makes it impossible to access Archived mail!
Is there any way to enable syncing to all folders? I'd rather have duplicate e-mails than limited access.
Also, it's not clear what the delete button will do. Will this send the e-mail to the trash (really deleting it from Gmail) or will it delete form the current folder, effectively removing the tag in Gmail and archiving it? There really should be a delete and archive/remove label button for the Gmail interface.
Also, I disagree that there needs to be duplicate e-mails. What I'd really like to see (I've been hoping someone would write a Thunderbird extension for this but I'd more seriously consider switchin to Zimbra if you do it first) is an interface that more closely matches the Gmail interface. So to avoid duplicate e-mails you could look at the message ID which should be unique. So really, the only folders that need to be fully synced is All Mail, Spam and Trash. These 3 folders should contain all E-mail messages. Then rather than using folders the interface should use tags, since this is what Gmail does. I don't know if this information could be attained through a Gmail API or otherwise IMAP can be used to sync to all other folders but you would only need to download the headers in order to get the Message ID. Then this could be matched to the same Message in the All Mail folder and used to add the appropriate tag. Folders on the left could be search links (for tag) as in Gmail or treated as real folders but each message would contain all synced tags so messages may be moved either by dragging to a different folder or manipulating the tags. Tags, of course, should be synchronized with Google. A local tag could be added to Google by creating an IMAP folder.
I did notice that conversation in the inbox did include messages from other folders, so it looks like Gmail interface is partially being mimicked, so why not go all the way?