If the CB configuration for 'emails sent from' is set to 'users address' the return address for the email gets set to the site email address as set in the Joomla system configuration instead of the users email address. If the config is set to "Admin Email Address, With Reply To Users Address" it works ok and you can return the email to the sender.
This appears to be a bug in 2.0.10 - in 2.0.9 both options worked ok and the email could be replied to correctly.