I've been using Zimbra desktop for Mac for quite a while to connect to our corporate zimbra account as well as to gmail.
I am pretty happy about it, but there are two major issues that really bother me. One is here, and the other is in my other thread
When I receive a message send by someone using another client (e.g. thunderbird) Zimbra does not display any prefixes but simply indents quotations. This makes it very hard to read and interpret long email discussions... Does anyone know how to fix this?