Shared Mailbox : Using inactive users' mailbox

When an active user is deleted, their data is automatically deleted after 30 days from Microsoft. IT admins sometimes assign the inactive user's alias to an active user, or download the .pst of the mailbox before un-assigning the license and deleting the user. However, there is a better way to retain mailboxes of inactive users while still keeping the email alias active for communications. 

For AD Connect Setup:

- Move the AD user to a non-syncing OU, this will delete the user in O365
- Restore the user in O365, this will turn the user into a cloud user

For Cloud Setup:

- In Exchange Online, convert the user's mailbox into a shared mailbox and assign delegates
- In O365 Admin Center, remove the user's licenses
- The unlicensed users should not be deleted or the shared mailbox associated with their alias will also be deleted