Since a HS user can create any email to be used to send tests to, we found that some of our users in the past had created bad emails. As such, those bad emails remain as a choosable email to use even though it was not and can never be validated.
I figured out how to remove these bad emails. Under the Inbox tab, and in the From Address box, click and choose “Manage email addresses”. Find the email you’d like to delete and select the trash can. There will be some emails that you can not delete because those are connected to a valid HubSpot User.