API currently doesn't expose email addresses of non-Box uploaders who upload via email
Specifically in the context of uploading via email.
API currently does show emails of uploaders that are Box users, whether in enterprise or not. It does not show emails of uploaders that are not Box users (name shows as "Unknown User".
Webapp shows names for Box users, email addresses for non-Box users. It seems odd that the webapp would expose the email address but the API doesn't.
A field called ‘uploader_display_name’ is now available on the file object via API and documented at https://developer.box.com/reference/resources/file/. This field should contain the email address of users that upload via email or the new file-request capability. Read more about the latter at https://support.box.com/hc/en-us/articles/360045304813-Using-File-Request-to-get-Content-from-Anyone