Select the "Sent" folder and click in the message you want to see the logs.
In the "More" drop-down select the "Message logs" option. This will open a new window with the table of your message recipients where you can check who has read or deleted the message.
If you see a red dot before the recipient name it means the user is inactive. Therefore they won't be able to access your message because inactive users can't log in to Assemble.