In order to get the ‘OPENED’ status we must receive a message back from the email client, for example Outlook, Gmail etc, to say that the email has been opened by the recipient - as shown below:
If the recipient has images on their email turned off – or they do not download the images contained in the email then we might not get the message back from the email client to say that the email has been opened.
In this case the email will still be shown as 'SENT' as shown in the example below:
If the recipient has download the images in the email then you will see that it will be updated to say that the email has been opened.
The behaviour is dependent on the email client that is being used – unfortunately this is not an exact science – due to privacy settings that have been incorporated into email clients over the years.