Forum Discussion
What's the best way to handle when an employee leaves the company? OneDrive and Email access
To preserve all of the emails I would do what others have said and set litigation hold on the mailbox. That way you'll be able to search the mailbox as an admin even if you delete the AD object and the license.
For mailboxes that need to be converted to shared mailboxes, they do still need the AD object and they are limited to 50GB and do not include an archive.
One way to get around the AD object tie is to delete the AD account. Let AD Connect Sync and you'll see the mailbox go into the Deleted Mailbox recycle bin. Wait a bit and restore the mailbox from O365. This will convert it to a cloud mailbox that no longer ties to the AD object. Keep an eye on that mailbox though as I've seen the sync try and delete the mailbox again, but it will stick. I've done this with mailboxes I've converted to shared and mailboxes that still required an Exchange license to do the size.