If recruiter gives the following answer: "Sorry I receive 400 emails a day so I am not able to reply" is that a proper reply to (potential) employees?

Definitely no.all applicants applying to a job vacancy is with different aspiration of the role and a company's  brand name looked up to,Certainly recruiters face huge volume of applications in the Middle East and its next to impossible to reply to al the applicant with their feedbacks.

in many companies there is well comprehended and positive auto reverts set stating we appreciate your interest and thanks for applying for the job vancay in our company looking at the huge number of volumes of applications we receive everyday we might not be able to contact you with the feedback our recruitment team will call you If your profile suits the set parameteres.

in the meanwhile we would urge you to kind,y keep visiting put website for the open vacanicies and ve aged your resume in our data pool for our recruitment team to call in for our future needs.

thanks once again for visiting xyz company.we wish you luck.

these answers would not demoralize the momentum of potential employees 

absolutely not,  it's an excuse for not doing the job right,

1) Organize the recruitment process by having the candidates to fill-in the application on lime .

2) Selection process by sort candidates for the right job position.

Improper, Unprofessional.

Recruiter or HR staff should be more understanding, humanely, patient and professional toward the applicants.

No, it is not a right answer for the HR Professionals.  It definitely damages the EVP and eventually losing employment brand value. In such case they have to review their recruitment and sourcing strategy.

agree with mr. Nuridin Islam Diab 

No, it's not a proper reply. Recruitment officers should set an automated instant reply to any email they receive from potential candidates stating what means that their resumes will be reviewed within the next two weeks and only short listed candidates will be contacted for further steps. This way there should be enough time to open emails and review cvs before they decide to take the next step with them.

No it is not! Answer could be tailored in other proper ways such automated reply, or by asking employees from the beginning to not send emails because successful candidates will be directly contacted ... etc

Thank You