It was confirmed that the email address requested during registration, is used for "passcode" authentication through email and not for Starling.
Password Manager does not write this email address to AD for security reasons, in case some one was to leave the company they could potentially change their Password using a personal e-mail account if that was used.
Starling does require a user's e-mail address in AD, so when implementing Starling email addresses must be populated before hand.
The product team will evaluate the request and this feature may become available on a future release of the product. Please refer to this article for future updates, or contact support referencing the Enhancement Request ID VSTS106412