A protected mode exists for the Thru Dropbox™, that requires external senders to self-register and then authenticate to upload files to a user’s Thru Dropbox™.
When clicking on a protected Thru Dropbox™ link, a login/registration page appears
See Secure Message Login – Registered Recipient Login for further details
Once registered, the From: field is populated with the registered sender’s information automatically.
For more information or if you wish to have this enabled on your site please contact firstname.lastname@example.org