EMail Out window's Search

I type a key(starting letter of an user in the shared names) and click the Load Shared Author email ID. It loads the remaining words of the author.

But it limits only with the username.

I thought username is enough for sending mails and it will get mapped inside, coz this is only a shared user.

But by clicking the send button shows the below error message

"Please enter a valid email address!"

Please clarify me if i am wrong.