If the employee's email was invalid, you would have received an immediate notification that the email "bounced". 

 

To be sure that the employee actually saw the message:

    a. Open the leave record.
    b. From the gear menu, choose View Audit Log.

    c. Scroll down to find the Email Sent entry with the appropriate Subject line.
    d. Below the subject line, look for the line that says First Opened,
        (with a date and time stamp), like the one shown below.
        If there is no such line, the email hasn't been seen.