If you find that an employee has left and you wish to keep hold of there emails. You can convert the mailbox to a shared mailbox which then releases the License to use with a new employee.
To convert mailboxes in the Admin Center, open the Exchange Admin Center in your browser. Select recipients then select the mailbox you need to convert. Click Convert on the right side of the screen, under Convert to Shared Mailbox header.
It will take a few minutes to convert the mailbox to shared. Once finished, remove the license from the mailbox in the Office 365 Admin center. Select the account, then click Edit next to the Assign license information on the right
. 
Deselect the licenses then click Save.
