Also known as:
What to do when an employee leaves and you want to keep their email but you don’t want to pay for a license anymore? And maybe you want to forward their email to another person in the organization?
Microsoft has good documentation on this, but if your prefer to TL;DR like me:
- Go to the Exchange admin center.
- Select Recipients > Mailboxes.
- Select the user mailbox. In the Mailbox tab, under More Actions, select Convert to shared mailbox.
- Back to Normal admin https://www.office.com/?auth=2
- Teams & groups
- Edit shared mailbox and add forwarding
Now contact your license provider and tell them to adjust billing.
Reference