It is possible in some cases not to receive a read acknowledgment or receipt or read.
This does not mean that your message has not been delivered, opened or read. It just means that the receiver's inbox does not support these settings.
For example, if you request a read or receive receipt to a Gmail.com account, confirmation requests will not work because Gmail has chosen not to manage this type of parameter.
When the recipient opens the email you sent them, depending on their settings, they can choose to send you a read and/or receipt or not. In this case, you will not have an accused.
Pour toutes questions, contactez le support