kernel version not really relevent (but did the 2.4 releases have nptl?). the problem is you're trying to install on an unsupported os, rhel3, whereas only rhel4 is supported. can you upgrade to rhel4 or another supported os? i presume you've downloaded the compat-libstdc++ packages from somewhere else, as they are effectively duplicates of the currently installed libstdc++. this is very strongly not recommended, but you could try using --force to rpm -ivh if you reaaaally want to install it
still, each binary release of zimbra is tied closely to its underlying os, and it does attach to several critical system libs such as ssl, libstdc++, libc, gssapi etc. i've tried forcing zimbra onto slightly nonstandard platforms and even if you get it running often find subtle errors that later become quite annoying.