Seeing this in mail log now. Even though the email address is correct!
Nov 23 17:20:38 server2 send: delivery 206: failure: User_and_password_not_set,_continuing_without_authentication./154.34.342.201_does_not_like_recipient./Remote_host_said:_550_No_such_person_at_this_address./Giving_up_on_154.34.342.201./