It is not mentioned in the API docs that you can not send a private message to somebody who does not follow you. (https://dev.twitter.com/docs/api/1.1/post/direct_messages/new)
Also this error code is not listed on https://dev.twitter.com/docs/error-codes-responses.
Is there any possibility to send a private message so somebody who does not follow me? (E.g. after he mentioned me in one of his tweets)