Sending feedback emails to candidates after conducting interviews must be a mandatory step in your hiring process if you are looking to improve the overall candidate experience. Sending Negative Feedback Mails should be a part of this step too as most companies ignore the candidates once they are rejected from the further hiring process.
Subject: Interview at [Company_Name] for [Job_Title]
We appreciate you taking the time to apply to [Company_Name]. We would like to inform you that we have finished conducting the interview and we have chosen to take a step forward with another candidate for the [Job_Title] position since we feel that his/her qualifications and the work experience he/she possesses were most suitable for this position.
We were impressed with your (e.g. performance on the assignment / in-person communication/experience in X tool) and we would like you to apply in the future when there is a job opening suitable as per your profile.
It was a pleasure getting to know you and we wish you all the best in your future endeavors.
Thanks again for showing your interest in [Company_Name].