I have seen conflicting responses on this topic over the years and I want to confirm the right approach. 

I have an employee who left. Her IMAP configuration was active. I do not want to lose her email history. It appears from current help documents that I can back up her emails before deactivating her, however in the case of other recent deactivated employees the button for "back up emails" is grayed out. [SEE SCREEN SHOT] I'm nervous about deactivating without being 100% sure that the emails will be backed up.