It's my pleasure to assist you, @accounts36.
For you to make sure that your P45 has been sent through an email, you'll need to contact your employee and let them confirmed that they received the email.
Currently, there isn’t a report that shows the email sent to your employees or received by them. You can resend your email if your not sure your employee has received it.
You can do it by following these steps:
To learn more about how you can add, edit, or inactivate an employee, you may use this article as your reference: Add, edit, or inactivate an employee.
Feel free to get back here in the Community if you need further help with managing your employee's payroll. Stay safe and have a great day ahead.