I've understand why I've got
zimbra@zbox:~$ cat /opt/zimbra/log/mailbox.log |grep nas
java.io.IOException: Unable to rename /mnt/nas/zimbra/store/incoming/1241001684562-0.msg to /mnt/nas/zimbra/store/0/3/msg/0/340-800.msg
it was simply a right permissions on mounted external nas volume. When zbox machine starts and mount in sambafs external nas volume with (/etc/fstab)
[...] said: 451 4.0.0 Temporary message delivery failure try again (in reply to end of DATA command)
it gives to local mounted volume on zbox right credentials zimbra:zimbra, but these are different from local ones because gid and uid of zimbra on zbox are different from xxx.xxx.xxx.xxx zimbra gid and uid. So i've done something different remounting external nas volume with right stuff and restart zbox machine.
//xxx.xxx.xxx.xxx/nas /mnt/nas smbfs user,defaults,credentials=/root/.smbpasswd,uid=zimbra,gid=zimbra 0 0
Now when I switch index1/message1 (local machine) to index2/message2 (nas storage) message are correctly delivered.
Something I'm still wondering ... when I log in into a client , how can I see all my stored old messages if i switched from store1 to store2 ? In theory I should see messages arrived from switching on and turning back to store1 messages arrived from switching back ... Am I mistaking?
There is a way to keep synched store1(local) with store2(nas) , I've to schedule a cronjob with rsync probably?
Last but not the least: Why my index2(nas) is still empty even if I've switched from index1(local) to index2(nas)? Is zbox machine still searching message indexes in index1(local)?