I believe the email BC use to send emails is the one from the user card. I don't have example, but believe, AAD create a long email address for the guest user which might not be able to receive emails. it is for authentication purpose.
If you are asking if its possible for a BC user to send emails from BC to extern/guest users then that's definitely possible.
Valentin Castravet
Zander ERP Services
Under review
Thank you for your reply! To ensure a great experience for everyone, your content is awaiting approval by our Community Managers. Please check back later.