Companies are spreading job posts to hire one employee, and they are receiving so many CV's, do you think it is from diplomacy to send an apologize letter for those who are not approved?

Yes, sending a regret letter to candidates who are not selected are one of the act in HR as professionalism, also for saving time from the  follow from mail or telephone, regards.

It create good impression if company send apology letters.

Courtesy is the coin you can never have too much of!

Sending Regret Letters to unsuccessful candidates is a courtesy gesture and must be followed after the position is closed!

In responding to unsuccessful candidate, it is not so much diplomacy but good ethical/business practise to do so.  By replying it will provide closure on the applicant's case allowing him or her to move on in their job search.  Secondly, the company will gain much reputational capital by demonstrating that it take great pains to respond to people who showed interest in their organisation.

ch candidate shortlisted may be the lucky ones because,the number of good candidates require by the company may affect the selection too.lets see where a company needed to recruit 4 managers and end up with 4000 applicants and out these applicants 400 are good but 100 were shortlisted.for interview.candidates who were not shortlisted can be give future hope in herethere search by sending a thankyou letter for all there effort in trying to get a job with the organisation for future trial.

I wouldn't choose dimplomacy as the word but rather courtesy. If a candidate has applied for a certain position and hasn't been selected, they would like to be informed about it. However, on the companies behalf its a policy that must be followed for many reasons.

1. Escape harassment calls from potential/ not selected candidates .
2. Keep up the good name of the organisation.
3. Basic humanity, you would like to be informed as a recruiter if you were selected or not.

Normally, the organization inform through advertisement that only shortlisted candidates will contacted for interview.  So, It is to be assumed that when someone is not contacted, he has not been shortlisted.

After interview, one or two persons are selected based on vacancy. So, as a courtesy  and to maintain goodwill, the  organization shall inform other shortlisted candidates in polite manner that he was not selected. 

It may not be very practical to inform all the applicants by email or letter when the number of applicants are huge. 

i think that sending an apology letter to those who weren't approved will work in the benefit of the company, because it will show that the company cares about it's employees and recrutements which can only help give a positive picture on the company's reputation  

Yes, I do, how you treat potential employees either successful or unsuccessful helps determine a company’s reputation for example an unsuccessful employee who receives an appropriate communication thanking them for their application and interest who may have been unsuccessful on that occasion may well prove to be successful for another role at another point in time, the employee in this process will talk highly of their treatment by the employer despite their lack of success to securing the role.  Opportunities to enhance employer reputation should always be taken, let’s assume the company was a financial institution the employee and their extended family may well bank with that institution and thereby it is clear to see the positive impact this good practise can have.

yes it is. 

To get a positive or negative answer in order to keep searching others jobs or not.