Yeah, good as well. I just wanted to make clear that forcing Zimbra to do a full backup by deleting the old one is dangerous.
And what Erwan meant too is that if you delete a full backup, the next incremental you do will be actually a full backup, not just incremental (same "problem" as adding a new user).
Exactly what we do /opt/zimbra/backup is an NFS share from another Linux server.
But you can obviously do a full Zimbra backup each night, export it (SAN, NAS, whatever) and use a third party tool (even rdiff-backup maybe ?) to handle the diff mechanism and do some "true" backup on disk.