Email is part of data that is encrypted.
$this->enc_data['customers'] = array (
'id' => 'customer_id',
'fields' => array (
'telephone',
'fax',
'email'
),
);
Are you sure you not confusing with login field? By default login will be same as email, unless it is configured to use login name
You can force login to be changed for customers that have that.