@Ashwin Malhotra, Thanks for posting in Q&A.
From your description, I know you are looking for a way to unregister the mobile device from previous organization without accessing to company portal.
For iOS device, here is the method you can refer to.
Setting>General>VPN & Device Management
Remove the account from there.
For Android device, here is the method you can refer to.
Settings>Privacy Protection>Special Permission>Device Admin Apps
After that remove your email or uninstall company portal and outlook.
Hope above information can help you.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.