SSRS Subscriptions "To" Field Not User Email

Hi, I know this has numerous other posts, but I could not find crystal clear, satisfactory answers in any of them.

SSRS 2019

When a user subscribes to a report in SSRS, the "To" field is pre-populated. However, it is pre-populated with their username, not their email. In our case, the username is part of the email, e.g. "[email protected]". I tried altering the SendEmailToUserAlias attribute in the rsreportserver.config file, but changing this to False merely makes it editable, and we don't want it to be editable. We want the email to show up. (If it is editable, users could have control over the email a subscription it gets sent to, and the point of the feature is for them to be able to set up subscriptions for themselves.)

(Mine does let me edit because of elevated permissions, but "To" is appropriately greyed out for standard users)

Changing the DefaultHostName attribute doesn't seem to either update the pre-populated email nor does it seem to constrain the domain to which emails can be sent (which could maybe be okay if we really couldn't get the true email pre-populated, but isn't desirable either). I put the same host name that is used in Report Server Configuration Manager for the sender address, which is on the same domain as user email addresses.

In AD, the userPrincipalName is the full email address. The only attribute that seems to have just the username is sAMAccountName - is this indeed what SSRS is pulling from? I would rather not have to change this attribute, but I might be willing to if it meant that users could subscribe to emails themselves automatically.

Is there really NO way to have the "To" field pull in the users email address?