First of all
I installed the zcs-5.0.10_GA_2638.RHEL4.20081003035619 on CentOS4.6
I fixed this issue by lowering the heap size :
This is what I got in the log file /opt/zimbra/log/zmmailboxd.out
The specified size exceeds the maximum representable size.
Could not create the Java virtual machine.
Invalid initial heap size: -Xms4872m
Our mail server has 16 GB of ram.
Then I lowered it to 10% using this command
zmlocalconfig -e mailboxd_java_heap_memory_percent=10
restarted zimbra and all okay now ...
p.s. thanks for all the help in here.