The system is equipped with a message monitoring module. If the message is not delivered (eg the e-mail address of the Customer is full or does not exist), the system will send you a notification.
In order to check the history of messages sent, log in to the system and go to the Clients / Transactions / Transaction List tab. Go to the details of the selected transaction and select Message history from the right menu. In the table you will find a list of sent messages to the client along with their status (read, sent, undelivered).
If the message with the code has been read by the customer, the field Number of sent codes will be highlighted in blue in the list of transactions.