I try to configure a zimbra-proxy (http,imap,pop).
I have 2 servers, 1 MTA/Proxy (mta.1.ne.domain.fr, mail.domain.fr) and 1 mailstore (mbox.1.ne.domain.fr) for the moment.
zimbra-proxy is enable on MTA/Proxy and disable on mailstore.

On my zimbra-proxy, i have this :

zimbraMailMode: http
zimbraReverseProxyMailMode: mixed
zimbraMailReferMode: reverse-proxied
zimbraReverseProxyLookupTarget: FALSE
zimbraReverseProxyMailEnabled: TRUE

On my mailstore, i have this :

zimbraMailMode: http
zimbraReverseProxyMailMode: http
zimbraMailReferMode: wronghost
zimbraReverseProxyLookupTarget: TRUE
zimbraReverseProxyMailEnabled: FALSE

The proxy works, but sometimes i have some timed out (no responding) and i have always this messages in my nginx.log :

2009/08/11 13:08:10 [error] 30301#0: *20 upstream timed out (110: Connection timed out) while reading response header from upstream, client: X.X.X.X, server: mta.1.ne.domain.fr, URL: "/service/soap/NoOpRequest", upstream: "http://X.X.X.X:80/service/soap/NoOpRequest", host: "mail.domai.fr", referrer: "https://mail.domain.fr/zimbra/"
I have look those threads