If you add a mailbox as IMAP in Outlook with the standard send/receive settings you may see a pop-up stating: "Your IMAP server closed the connection. This can occur if you leave the connection idle for too long." Usually this occurs if multiple users are accessing the same mailbox frequently.
I am posting this to assist those searching for a fix for this issue. It took me a while to find any solution and the users are extremely grateful that they are not receiving the pop-up every few minutes. Since the server postfix configuration is not modified this should be considered a client work-around rather than a solution.
The IMAP connections are live connections and will receive the mail regardless of your scheduled send/receive settings. If you create a separate send/receive group in Outlook (Tools -> Send/Receive -> Send/Receive settings -> Define Send/Receive groups...) for the IMAP mailbox you can avoid the pop-up by de-selecting the option to “Schedule an automatic send/receive every X minutes.” This sends a NOOP request that works great for Exchange but is not a standard amongst all IMAP implementations, ie Zimbra postfix.
I use a combination of the Zimbra Outlook connector (user mailbox) and IMAP (general shared mailboxes) to allow the ability to drag-and-drop emails AND FOLDERS to and from the respective mailboxes, and download only headers when available. This is also a way for users to manage their own archive mailboxes (additional mailbox license required I believe) on a separate Zimbra mailbox server.
And in case you are wondering why we use still use Outlook.. We have an internal application that requires the ability to drag messages from the email client into the application. Zimbra desktop and webmail do not have this ability. Once the application is updated to integrate with Zimbra directly we will do away with Outlook and switch to Zimbra Desktop or Thunderbird.
Hope this helps.