To be able to edit any details for an ex-employee, you will need to mark them back as current temporarily.
To do this, go to the employee profile>scroll down>employee status>mark as 'current'
PLEASE ENSURE THAT YOU MAKE A NOTE OF THE LEAVING DATE AND THE REASON FOR LEAVING BEFORE YOU CLICK 'UPDATE STATUS':
Once you have marked the employee back as current, you will be able to edit the email address.
Don't forget to mark them back as 'ex' afterwards!