User outside of group receiving emails

I have a user that is receiving emails from a group he/she is not a member of.

Troubleshooting Steps Taken:

- Adding and removing this user via the admin portal and Powershell CMD. User still receives emails addressed to the group.

- Verified no forwarding rules in the user directory

- User is not listed in the group directory or group list in the address book (verified)

- Trace performed, user is forwarded messages without error from sender (how is this possible?)

- Deleting the group and recreating is not an option (Estimated size is >4GB)