I currently have Zimbra working very well, with the exception of IMAP.
Our setup is like this:
Internet --> Firewall 1 --> Zimbra MTA --> Firewall 2 --> Zimbra Mailstore Server
Now, when i point an IMAP client to the MTA for IMAP, i get the message saying authentication rejected (regardless of if i use clear text or TLS - cleartext is preferred). If i point the client to the mailbox server for IMAP, it gives me a timeout.
IMAP as far as I can tell is enabled.
EDIT: Also I have forwarded the IMAP and IMAP Proxy ports (as well as 7072).