IMAP Folders Dont Sync?

[BigBirdy]

Running the latest Zimbra on CentOS 5.4. As I continue to evaluate the product, I have created a coiuple of accounts but they also have an IMAP account pointed to a local Scaliz server. I can run Thunderbird locally and connected to the Scalix instance and open either then web interface or Zimbra client, yet the IMAP folders do not stay in sync? I can delete a message in TB, but it still remains in Zimbra? This does not make sense. If Zimbra is simply connecting to a set of remote IMAP folders, it should remain in sync. It appears that Zimbra must maintain its own local set of folders, or something along those lines. How can I ensure that connections to remote IMAP accounts remain synchronized?