Sometimes you'll get a timeout error on Outlook when using the synchronized IMAP protocol. You can avoid the timeout error message "Your IMAP server has closed the connection" by changing the settings in Outlook.
That should fix the issue, or at least minimize the connection timeout error from coming up.
If the above didn't fix your issue, the problem may be that multiple users are accessing the same mailbox frequently. Since the server postfix configuration is not modified, you should consider this 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 (i.e. Zimbra postfix).
In less technical terms and under Outlook 2010, here's how to deactivate automatic send/receive for your IMAP email account (which already doesn't really need to be on a send and receive schedule. IMAP uses push technology.):
Theoretically, that should stop that IMAP account from being forced to send/receive when the connection has become idle. After all, IMAP doesn't really need to actively send and receive every X minutes. That's a POP3 thing. IMAP is awesome.