The feedback giver is trustworthy. The giver is sincere and can be trusted and respected. The feedback is conveyed with good intentions. The giver either has the receiver's best interests at heart, or it is otherwise apparent that the feedback is intended to serve a worthy purpose and is not self-serving. The timing and circumstances of the feedback are appropriate.
Effective feedback is timely and conveyed under conditions that are conducive for learning.
For example, it's conveyed at a time that the receiver is open to feedback. Also, most people prefer to receive negative feedback privately. The feedback is given in a personal and interactive manner. The feedback is given in person so the receiver can observe the giver's facial expressions and body language, and has an opportunity to ask questions and clarify the message. The feedback message is clear.