I have installed Zimbra on a Centos 5 machine.

It has ~4,5GB of RAM but right after installation i started to get errors related to memory.

[zimbra@mail1 ~]$ java version
Error occurred during initialization of VM
Could not reserve enough space for object heap
Could not create the Java virtual machine.
[zimbra@mail1 ~]$ free -m
             total       used       free     shared    buffers     cached
Mem:          4500       3686        813          0          0          0
-/+ buffers/cache:       3686        813
Swap:            0          0          0
It still doesn't have any "real accounts" only test ones without any activity at all.

Why exactly this happens? Isn't 4GB of RAM enough to run zimbra?