LAN IPs on Mail.app Email Headers

Dear Apple,

please explain why is my computer (added to not be mistaken with the router’s IP) LAN IP address showing on the email headers of the mails I send with Mail.app.

Thanks!

Bellow, in red, my computer’s private LAN IP Address. In green my Router’s Public/WAN IP address which is “normal” to be included on most email headers.

Return-Path: <rsaramago@gmail.com>
Received: from ?XX.XX.XX.XX? (pa6-XX-XX-XXX-XXX.netvisao.pt [XX.XX.XXX.XXX])
 by mx.google.com with ESMTPS id 7sm502355eyb.8.2009.11.13.01.07.07
 (version=TLSv1/SSLv3 cipher=RC4-MD5);
 Fri, 13 Nov 2009 01:07:08 -0800 (PST)
Subject: Teste
Mime-Version: 1.0 (Apple Message framework v1077)
Content-Type: multipart/alternative; boundary=Apple-Mail-1-623152288
From: Ricardo Saramago <rsaramago@gmail.com>
To: Testy McTest <test@test.pt>
X-Mailer: Apple Mail (2.1077)

Update: I’ve clarified some descriptions above after some user comments, I realized that It wasn’t clear what IPs I was referring to.

It seems that this is common on most email clients, except for Outlook. This “issue” triggered my attention when I was looking into the mail headers from a mail I sent from Mail.app in response to a mail from Outlook and they were indeed different in this aspect.

The client’s computer Local IP address and the Router’s / Firewall / Modem / whatever public IP address are added by the SMTP Server to the Envelop’s “Received” line, which it probably gets from the EHLO.

Still, this isn’t secure as it allows malicious attackers to map a victims network very easy.