Verify selinux is turned off with sestatus command.

See also Zimbra with Zextras Add-On

Zextras includes backup for OSE.

http://wiki.zimbra.com/wiki/Backing_up_and_restoring_Zimbra_%28Open_Source_Version%29

http://vwiki.co.uk/Zimbra_FOSS_User_Backup

Permissions before:

ls -al /opt/zimbra
ls -al /opt/zimbra/backup

Mount an NFS share:

vim /etc/fstab

# Mount Zimbra backup share
ip.of.nfs.svr:/path/to/backup/share        /opt/zimbra/backup      nfs     _netdev         0 0

mount -a
mount
ls -al /opt/zimbra
ls -al /opt/zimbra/backup

Reset permissions, if neccesary:

chown -R zimbra.zimbra /opt/zimbra/backup
find /opt/zimbra/backup -type d -exec chmod 775 {} \;
find /opt/zimbra/backup -type f -exec chmod 664 {} \;