Exchange 2013 reject email for not existed user in a mailing list

I have an Exchange 2013 server. Faced with a strange issue of email delivery what I can not solve. Exchange is rejecting email
if one of the recipients is not exist in a domain Email will be rejected totally and sender will receive NDR for not existing user. 

scenario:
Exchange mailboxes user@domain.com  // valid user
user1@domain.com  //**user mailbox is not exist**
Sending email from external domain to my exchange server:
TO: user@domain.com,user1@domain.com
Reciving NDR Remote Server returned '550 5.1.1 RESOLVER.ADR.RecipNotFound;
valid user didn't receive email also.
email is rejected

I start to google and found that   How do I reject incoming email for unknown users in MS Exchange 2013?exchange has a user validation feature that is enabled by default I disable it on a console restart transport role. Set-RecipientFilterConfig -RecipientValidationEnabled $false

run the same scenario again user@domain.com, user1.domain.com I receive NDR for not existing user. but email was delivered to valid user.

Is it a normal behavior for exchange?

What is right by RFC deliver for valid users or reject email if one of the user is not exist?

how it will affect our server hit the public spam list?

Does this not reduce our spam filter resistance?

Please advice.

Thank you.