What feedback should you provide to suppliers as a minimum (if any)?

I'd be interested in hearing your thoughts on what you think is best practice when it comes to providing feedback to unsuccessful suppliers, particularly when they've taken part in a long tender process? Is a simple 'you've been unsuccessful email' satisfactory? Should you provide additional feedback on why they were unsuccessful/suggestions for improvement? Should you provide any feedback at all? Are there any considerations (excluding time) why you wouldn't provide feedback?