There are a few reasons why an email address may appear within the Email Center within the Mails Probably Not Opened list:
A person may have received the email but has yet to open it.
Some email clients, especially on mobile devices, aren't recognizing or are blocking the email's tracking feature, or tracking has been disabled by the user. This causes the email to appear unopened, even if it was.
The user's email address is incorrect or needs to be added to their account.
The email was sent to their junk/spam folder and needs to be marked as Not Spam. In Gmail, check all category tabs for the email: Primary, Promotions, Social, etc. If it is still not found, they must check via the email host's web interface, not just the app.
As an example, a member with a Yahoo email address would log in to yahoo.com to check the junk/spam folder.
The user needs to add the notification email addresses as a "safe sender."
If the above fails, and the email host claims the email was not received around the date and time it was sent, contact Support by clicking the blue Need Help? button located in the lower left corner. If their email host claims they never received this specific email around the date and time you sent it, contact Support for investigation as to why. Please include the email subject line, the date and time sent, and the recipient who didn't receive the email in your message.