When you would like to check the status of an email you will want to open up that reservation then click on the log tab. Under this tab you will see the section called Email Activity. This will show a breakdown of when the email was sent delivered and opened. Below is a screenshot of what you will typically see for email activity.
You will also be able to see if an email fails to send and it will notify you if the email is invalid. In the screenshot below you will notice the notification which will show a message when you hover over it. In this example this is an invalid email.
This comes in handy when you have customers that reserve online and accidentally have a typo when filling out their contact information. Once the email address is corrected and saved you can resend the email in question. You will then see the status updated on the Log tab under the Email Activity section of a reservation.