How to configure RDS Web Services for Email Login.
Make a copy of the RDS web Folder before making any changes.
Go to the following folder on your RDweb Server.
C:\Windows\Web\RDWeb\Pages\en-US
Open the following file with a text editing tool like Notepad ++.
You will need to edit lines 19, 27, and 28 so that it shows email address.
Save and then exit
Search for the following file under C:\Windows\Web\RDWeb\Pages\webscripts-domain.js
Then make the following changes:
On line 44 add “//” at the beginning of the line.
on line 46 remove “else” so you are left with if as shown in the screen shot.
There you have it, RDWeb now configured for your users to login via the UPN (Email address).
To make things easy, I have created the files so that you can replace the default, you can download these from this address: Download here
Ryan Mangan's IT Blog 
I did this and when I go to login, it fails with “The Email Address or password that you entered is not valid. Try typing it again.”
Can it be any email alias, or is this strictly just for UPNs (user@domain)?