The first reason why the total number of Joyn customers is higher than the number of recipients of your emails is that not all Joyn users register their card.
To be able to send emails, you obviously need the email address of your customers. Always encourage new Joyn users to register or, better still, to use the Joyn app on their smartphone.
Whenever possible, try to enter the customer’s email address on your tablet when issuing a Joyn card to a new user. The customer then only needs to confirm the email address in the email he received from Joyn.
A second reason why the number of recipients of your emails is lower is that customers can unsubscribe from email campaigns. The trick here, of course, is to keep your emails as interesting as possible, so no one wants to unsubscribe.