Is it possible to append custom headers to message, ex, score calculations and final score (X-SPAM-SCORE: 100)

Although there is no way to add custom headers, the score is in the X-SMScore header.

I didn't find the header

X-LCID: 23534
Received: from [(xxxxx)] by xxxx with xxxxx SMTP; Mon, 28 Feb 2022 17:25:11 +0000 (UTC)
X-SM_EnvelopeFrom: xxxxx@xxxxx.xxxxx.com
X-SM_SENDER_IP: xxxxx
X-SM_HeloStrInEnvelope: EHLO xxxxxx
X-SMRecipient: xxxxxx
X-SM_RECEIVED_ON: Mon, 28 Feb 2022 17:25:11 +0000 (UTC)
DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; s=xxxxx; d=xxxxx;
 h=MIME-Version:Content-Type:Content-Transfer-Encoding:Date:To:From:Reply-To:
 Subject:Message-ID; i=uxxxxxx;
 bh=e+uSxc3f27nKwQyPTlpTfDHpZnRA0ZyXaVjg1mjvnMU=;
 b=hyDTDSkKWU0dFvP2fl3dQzk9nZ7wPzqKzzY/o2wIlgDEmaQzG3jzCmJMT4fJYHiF+1i+QACyNfs/
   wHLWCk/GyeoFasTHjW7AlNJQyq30XHTYVTT3wxWlwf3QPjldLNAHt6HePR6uET+msm/Mgvn5vkxE
   hE3TFd9GthFFPB3dE0A=
Received: by xxxxxx id h3k2n82q38ks for <xxxxxxx>; Mon, 28 Feb 2022 09:25:07 -0800 (envelope-from <xxxxxxx>)
MIME-Version: 1.0
Content-Type: text/html;
	charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Date: Mon, 28 Feb 2022 09:25:07 -0800
To: xxxxxx
From: =?UTF-8?B?VXB3b3Jr?= <xxxxxx>
Reply-To: =?UTF-8?B?VXB3b3Jr?= <xxxxx>
Subject: [PossibleSpam] xxxxxx
Feedback-ID: 51830:20598065:oraclersys
X-sgxh1: khrpJxtpNLgLlOhjskQus
X-rext: 6.interact5.EQ1C7bk3_yvCVmHJ-wjUCmKzVMMwNeZpbbkxjl8j1pdYHl7sxvR4EOnoPQte4-_cow
X-cid: xxxxx
X-ei: E-LuzAo1U0dGCE5JYhI2CXNYU7s
Message-ID: <xxxxxx>

My bad. This header is only added before sending the message to the next hop. In other words, it is only added when you run Xeams in Hybrid or Firewall mode. It is not added in Stand-alone mode. I assume you're using Xeams in stand-alone. Is that correct?

Could you please tell me a use-case why you need this header if all you're doing is to pull the messages using an IMAP/POP3 client? I can certainly submit a feature request for this.

I would like to see the score and the score calculations in my client. There are some plugins for Thunderbird (ex: spam-scores), or if needed to develop custom plugins.

Outgoing score is irrelevant. The receiver server will not take it in their calculations. 

BTW, modifying subject with [POSSIBLE SPAM] is also "bad practice" since there is client side DKIM validation, or if I validate the message (for any reason), it is modified and DKIM signature will be always wrong. It is better to add custom X- headers and the e-mail client developers to consider to allow to the users to customise which header to display.