We are running Zimbra 4.5.3 Network Edition on Sun x4200s and x4100s with RHEL4. We’ve fully patched Redhat and I’m 99% sure certain Redhat was installed with full 64 bit support.

# gzip -dc zcs-NETWORK-4.5.3_GA_733.RHEL4_64.tgz |tar xf -
# cd zcs
# ./install.sh
...
Checking for prerequisites...
    NPTL...FOUND
    sudo...FOUND sudo-1.6.7p5-30.1.3
    libidn...FOUND libidn-0.5.6-1
libidn-0.5.6-1
    curl...FOUND curl-7.12.1-8
curl-7.12.1-8
    fetchmail...FOUND fetchmail-6.2.5-6
    gmp...FOUND gmp-4.1.4-3
gmp-4.1.4-3
    compat-libstdc++-296...FOUND compat-libstdc++-296-2.96-132.7.2
    compat-libstdc++-33...FOUND compat-libstdc++-33-3.2.3-47.3
    /usr/lib/libstdc++.so.5...FOUND
    /usr/lib64/libstdc++.so.5...MISSING

###ERROR###

One or more prerequisite packages are missing.
Please install them before running this installer.

Installation cancelled.

#

The solution is relatively simple:

# up2date --arch=x86_64 -i compat-libstdc++-33

Fetching Obsoletes list for channel: rhel-x86_64-es-4...

Fetching rpm headers...
########################################

Name                                    Version        Rel     
----------------------------------------------------------
compat-libstdc++-33    3.2.3    47.3     x86_64


Testing package set / solving RPM inter-dependencies...
########################################
compat-libstdc++-33-3.2.3-4 ########################## Done.                   
Preparing              ######################################
##### [100%]

Installing...
   1:compat-libstdc++-33    
########################################### [100%]
#

Redhat doesn't name their 64 libraries differently than their 32 bit libraries. I expect there's a way to tell the difference.