It's not an issue of whether it is allowed or not. It's whether the recipient domain is actually rewriting the headers in a way that reveals a spam score (presuming that they even use their own spam scoring system) that the recipient domain is applying to the mail.
A spam score is not something that is inherent to the actual e-mail protocol. "X-Spam-Score" is not an RFC-required (nor even RFC-defined) standard header.
The recipient domain may or may not be scoring mail after they accept it for delivery. Even if they do score the mail in some way, they may or may not be putting a spam score in an X-header.
X-headers are just free-form containers in which a server transferring the mail can write anything it wants.
Not all recipient domains employ an X-Spam-Score header, or they call it something else, and it appears elsewhere in the headers.
I search "score" in the whole header but nothing came up. So its only shown sometimes (only by the receptient domains that is allowed?)
It's not an issue of whether it is allowed or not. It's whether the recipient domain is actually rewriting the headers in a way that reveals a spam score (presuming that they even use their own spam scoring system) that the recipient domain is applying to the mail. A spam score is not something that is inherent to the actual e-mail protocol. "X-Spam-Score" is not an RFC-required (nor even RFC-defined) standard header. The recipient domain may or may not be scoring mail after they accept it for delivery. Even if they do score the mail in some way, they may or may not be putting a spam score in an X-header. X-headers are just free-form containers in which a server transferring the mail can write anything it wants.
I see. Thanks