I am running zimbra in a xen domU environment and there's no nptl in the debian 3.1 version. Therefore I have to use etch. (Unless there's a way to get nptl running - didn't find a easy one though
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As Debian 4.0 has been released it'd be a smart move to release 4.0 zimbra binaries. Debian is one of the most used and most stable linux server distros around.
I wouldn't worry about the missing rpm command - there's none. Debian's using apt-get/dpkg for package management!