How to retire a ZohoMail Add On to CRM identity and keep email history?

I have Zoho Mail Add-on for 3 people in my company. One them retired from the company. How do I keep all of the email of "retired" person in our CRM system? If I delete the user to free up the license, I'll lose all of their email. It appears that I'll have to keep paying for this email address forever to keep the email history. Is this correct? If so, it seems like the implementation of this feature needs to be redesigned by having all email that is be shared move into its own storage area inside of ZohoCRM not ZohoMail.

Please confirm best practices for this situation.

Thank you.

Eric K.