I don't know if there is an authoritive teaching of the Church on it, but in my personal opinion, I do not think it is valid. The Eucharist is the ultimate symbol of unity--it is the glue that bonds us together as Orthodox Christians. It is how we are joined to Christ. There is only one Christ and that Christ is not divided, and there is only one proper faith about that Christ. I do not think that people who are not a part of the Church can legitimately prepare and receive the Eucharist and be united to Christ--that's a privilege and great responsibility that God has only bestowed upon us Orthodox Christians who adhere to the proper faith and have the authority via apostolic succession.