Hello,
Sorry but I don't see the point and clearly don't see any relation with HikaShop and our support.
Please understand that your question is far beyond the e-commerce subject and outside the limits of our support.
But I'll give you a little information :
Yes, spamassassin is giving you a bad point because you're doing something that spams do ; the reply email is not in the same domain that the "from".
And if you read the Nicolas' message you'll see that he did not tell you to use another domain but he told you that you can use two different emails addresses ; specially because you never mentioned that and it's a common knowledge that it's not good to send email like spammers do.
It's not a problem to have the reply-to email address different than the from email address as far as I know
For example, no-reply@mydomain and joe@mydomain are two different emails addresses.
So yeah, you can use a different email address but if you do not want to loose point ; using the domain name of your server it far better !
Afterwards, I'm afraid that we won't be able to help you further for questions regarding spamassassin.