What to do.....

You apply to a job.

You get a phone screen.

You do well enough to get an interview.

You think you do well at the interview, but, they send you an email and tell you that they are hiring someone else.

Do you mail back and say something like 'As I'm constantly trying to improve myself, may I ask what shortcomings you feel I had so that I may be more valuable to other employers,' or do you just delete the mail and move on?