Emails sent to contacts via email clients such as Outlook, Gmail, iMail etc will be shown as 'Email' inside your contacts history record as shown below: 

But they will not show a message status such as 'SENT', 'OPENED' or 'CLICKED' etc as they are sent via your email client (with a copy stored inside your CRM account as shown above). 

Email sent out via the CRM internal pop up email service are sent out over the e-Marketing platform and so is a different route to those emails sent from your email client.  

Therefore you will see a mail status such as 'OPENED' alongside any emails sent out have been sent via the internal pop up email service as shown below: