When on a lead and looking through the log to see if a customer has received a sent email, we provide a "read" date and time. Most email programs prevent us from knowing exactly when an email is read as a way to prevent spam.
Consequently, this information is not always accurate. However it can still be useful since in most cases if it says it is read, it truly was received. We say in most cases, because now some email servers will read emails to look for malicious content and sometimes that will trigger us to mark it read even if the customer never opened it. We try to prevent this, but it isn't always possible.