Once an employee's record has a leave date added and the pay run completed, you can't undo this.
Create a new employee record for them. Treat them like any other person who starts work at your company.
When saving the new employee record, you can see the following message:

This happens because the original employee record still contains the original email address.
It isn't possible to delete the original email address from the original employee record.
Follow one of the solutions below.
The major email suppliers (Outlook, Gmail, Hotmail) support this tweak.
Insert +1 before the @ in the employee email address.
[email protected] becomes [email protected].
The email address will work the same as the original.
The employee doesn't have to change anything within his email settings.