Further checking the LMTP conversation by using tcpdump -i lo port 8024 and the CC header still appear as is during LMTP:
220 list.host.domain GNU Mailman LMTP runner 2.0 LHLO list.host.domain 250 list.host.domain MAIL FROM:<ktso@myhost.domain> 250 OK RCPT TO:<listname@listserv.domain> 250 OK DATA 354 End data with <CR><LF>.<CR><LF> Received: from myhost.domain (myhost.domain [192.168.123.123]) by list.host.domain (Postfix) with ESMTP id 9370C155349E for <listname@listserv.domain>; Fri, 22 May 2020 09:53:58 +0800 (HKT) Received: by myhost.domain (Postfix, from userid 1000) id 6D848320BA5; Fri, 22 May 2020 09:53:58 +0800 (HKT) From: ktso@cuhk.edu.hk To: listname@listserv.domain CC: "ABC DEF (XYZ)" <ktso@cuhk.edu.hk> Subject: testing please ignore Message-ID: <sYGHb7tfH7549ub3To987sd@9oHHH7934v.8shy65FGvsd> Date: Fri, 22 May 2020 09:53:58 +0800 (HKT)
hello . 250 Ok QUIT 221 Bye
Since the CC header remains unchanged if we test using "mailman inject", we may further check whether Mailman LMTP Runner change the CC header.