Afterwards, open the «Sent» folder. There will be a tracking status icon near the message you just sent. If it's gray , the message has not been read by anyone. If the icon is green , at least one recipient has opened it.
Hovering over the icon allows you to see when exactly the message has been read.
If you open the message thread and hover over the tracking icon , you can see detailed information on who exactly has read this message (in case of several recipients) and when.