You could aggregate the external LUNS via Linux LVM to create larger logical volumes.
1. This is more likely an OS limit rather than Zimbra limit, and you can add additional volumes when the initial set of disks runs out. But it sounds like you are using ESXi since you mention vCPU (I'm using 2TB iSCSI targets in an ESXi environment too), which means you are bumping into the ESXi 2TB iSCSI volume limit, rather the RHEL or Zimbra.
vgcreate mailgroup "LUN1" "LUN2"
lvcreate -l 100%FREE -n mailvolume /dev/mailgroup
later you can add "LUN3" and do an online grow of the mailstore.
vgextend mailgroup "LUN3"
lvextend -l 100%FREE -n /dev/mailgroup/mailvolume
Where "LUN1","LUN2" and "LUN3" should be substituted with actual device paths
8.x NE + OSS KVM guests and Physical servers Ubuntu 12.04, RHEL 5 + 6 .
on Dell server hardware.
Previously: ZCS OSS and NE 7, 6, 5, 4, 3 variously on physical and virtual