Give honest feedback that will help the candidate understand your environment. Of course you have to be aware of legal implications but letting a candidate know as an example that they do not have experience in an area your company was seeking will provide an understanding of why they were rejected. This is far superior to them forming their own opinion. Perhaps you discover the person doesn’t show something on their resume and you mention that and they then explain they didn’t want to make their resume to long so they didn’t provide that information. You might lose a solid candidate because you didn’t express what you believed to be true. Dig deep and make sure you ask all the questions to make a good decision.