Do you want ZWC to be your default mail handler when you click a mailto: link on a web page?
Want ZWC to be used as the mailer when you use the Send To command in MS Office?
I don't personally use any of the following (Mac Mail is my primary client) but I thought they sounded promising and might be of interest to users of ZWC. Would like to get feedback from anyone who tries them.
Affixa - Attachments Made Easy - Gmail, Yahoo! Mail, Outlook, Thunderbird, Windows Mail
GmailDefaultMaker Note, in spite of the name, it supports more than just Gmail, although you might need to bug the developer to get Zimbra support.