Enable ARC for Email Security

Hi ElieAT 

>if we put the domain name in ARC it will whitelist all the emails including this domain?

No that's not how ARC works. 

If the 3rd Party Mail offers ARC Signing you can add the Trusted ARC Signer Domain.

https://blog.icewolf.ch/archive/2022/12/30/authenticated-received-chain-arc/

Have a look at a Mail that has been sent to Junk or Quarantine and use Message Header Analyzer https://mha.azurewebsites.net/ 
Check Authentication-Results and see if there is an ARC-Seal.

Regards
Andres