I was looking at the article in the wiki for setting up the IMAP & POP proxy with the MTA on one server and the mail store on a separate server. Does the open source edition of Zimbra support this type of installation? I would also like to know if such a setup is worth it for security alone. Assuming you had the mail store behind a firewall with rules to only allow traffic to and from the proxy/MTA server. Also have the proxy/MTA server with all internet facing ports blocked except https(webmail), SMTP SSL(465), and IMAP SSL(993).