In the Activity Feed at the very bottom of each job details page, you'll see record of all actions done on the job. Which employee completed the action and when it was done will be documented as well. Please check that your email was sent to correct address.
Look through the list to find the item/email you're looking for (ex. 'customer receipt email sent to firstname.lastname@example.org'). Click on the downward arrow to the right of the item to see all of the details about that specific email*.
Any emails that have not been delivered will appear in red on the activity feed.
*Some email providers do not share whether or not the email was opened and/or clicked.