Well if you aren't using the samba/posix stuff for anything, then you probably want to remove it.
Try running these commands as the zimbra user and see if you get any errors, if not, see if it got rid of the other errors you are getting about posix attributes:
Code:
zmprov mcf -zimbraAccountExtraObjectClass posixAccount
zmprov mcf -zimbraAccountExtraObjectClass sambaSamAccount
You might also have to do some removal of the samba/posix specific attributes.