Here you can learn what happens with your custom domain addresses when you delete the custom domain, or when you downgrade to the free plan.
The domain’s addresses that have been used to send and/or receive emails will become orphaned addresses. These orphaned addresses cannot be used to send or receive emails, but we keep them and their keys around so that you can still decrypt and read emails associated with them.
You can find orphaned addresses in Settings → Identity & addresses → My addresses → Status column.
Since their domain has been deleted, orphaned addresses cannot be enabled. However, if you add their domain again, they will automatically link to the domain and then you can enable them. As with all other custom domain addresses, you can only delete an orphaned address if there are no emails associated with them.