I believe that's an ADR (Additional Document Request).
If you're PNP Outland, I'm guessing you did your e-APR like a month ago.
You can use the web form here:
http://www.cic.gc.ca/english/contacts/web-form.asp
(the Tel Us More button on the left)
In order to do a CSE (Case Specific Enquiry) and upload the document they're asking you (
copy of bio page of your passport).
I believe that's the way to do it, as I don't think you can reply to that email.
Maybe anyone else could confirm this.
Good luck.