If one try to import a csv user list without an email column, the import fails. But having an empty email column will work fine.
So there is an issue here either the email is mandatory (based on what ? on the configuration ?) and an empty email should raise an error too, or it's optional in which case no error must be raised if the column is missing.
In addition, at least the lists generated by Claroline should always work fine for export-import of users between courses !