If you have EMail addresses that are forwarded to you, this is an example of a normal EMail message arriving after being forwarded.
Received: from Valid Relay Address Withheld by Your POP/SMTP Server's Address with ESMTP (Eudora Internet Mail Server 1.2); Fri, 23 Jan 1998 13:09:52 -0500 Received: from wabwall2.wabco-rail.com ([22.214.171.124]) by Valid Relay Address Withheld (2.0 Build 2119 (Berkeley 8.8.4)/8.8.4) with SMTP id NAA26418 for Valid Relay Address Withheld; Fri, 23 Jan 1998 13:14:40 -0500 Received: from darth.pei.wabco-rail.com by wabwall2.wabco-rail.com via smtpd (for Valid Relay Address Withheld with SMTP; 23 Jan 1998 18:09:41 UT Received: from PEIW95RMS.pei.wabco-rail.com ([172.20.12.131]) by darth.pei.wabco-rail.com (post.office MTA v2.0 0813 ID# 119-30594U510) with ESMTP id AAA345 for Valid EMail Address Withheld; Fri, 23 Jan 1998 13:09:39 -0500 From: Valid EMail Address Withheld To: Valid EMail Address Withheld Subject: lodges and guides Date: Fri, 23 Jan 1998 13:09:40 -0500 X-MSMail-Priority: Normal X-Priority: 3 X-Mailer: Microsoft Internet Mail 4.70.1161 MIME-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Message-ID: <19980123180939918.AAA345@PEIW95RMS.pei.wabco-rail.com>
In this case, you can see that a dial-in account on wabco-rail.com (PEIW95RMS.pei.wabco-rail.com ([172.20.12.131])) sent mail through various Mail Servers and then on to your Post Office. Notice that the way a dial-up is described here is different than that of ibm.net or pobox.com. Also, notice Message-ID contains the sender's domain, wabco-rail.com.
Additionally, From contains sender's domain and address.