"The user to send to could not be found" when using API, using an e-mail identifier

"The user to send to could not be found" when using API, using an e-mail
identifier

for example this query:
curl -H "Content-Type: application/json" \ -d '{"oauth_token":
"...","pin": "....","destinationId":
"reflector@dwolla.com","amount":"0.5","fundsSource":"..."}' \
https://www.dwolla.com/oauth/rest/transactions/send
returns the following error:
{"Success":false,"Message":"The user to send to could not be
found.","Response":null}
I also tried with a confirmed user account. I can send money using a
dwolla number ID, but when using an e-mail I see the "The user to send to
could not be found." error.
Why is this happening?
Thanks in advance