MTA is message transport agent. These are the boxes/programs that actually move the mail around, usually via SMTP (simple mail transport protocol).
The headers are exactly that, lines at the beginning of the email which explain where it's from and who's it to, etc. Each MTA adds to the headers as it handles the message. Here is an example:
Return-Path: <noreply@freshmeat.net>
Delivered-To: [removed]@[removed]
Received: from viruswall1.corona.navy.mil (unknown [137.67.32.41])
by zeus.corona.navy.mil (Postfix) with SMTP id AAEE970F29
for <[removed]@[removed]>; Fri, 4 Oct 2002 06:40:25 -0700 (PDT)
Received: FROM mail.freshmeat.net BY viruswall1.corona.navy.mil ; Fri Oct 04 06:41:14 2002 -0700
Received: from localhost.localdomain (localhost.freshmeat.net [127.0.0.1])
by localhost.freshmeat.net (Postfix) with ESMTP
id 7774A8341F; Fri, 4 Oct 2002 09:36:21 -0400 (EDT)
Received: from freshmeat.net (www4.freshmeat.net [10.2.35.4])
by mail.freshmeat.net (Postfix) with SMTP
id 771A18341F; Fri, 4 Oct 2002 09:35:31 -0400 (EDT)
From: <noreply@freshmeat.net>
To: <noreply@freshmeat.net>
Subject: [fmII] iftop 0.7 released (Default branch)
Message-Id: <20021004133531.771A18341F@mail.freshmeat.net>
...body of message was here...
You can see that this message originated at mail.freshmeat.net at 9:35EDT and reach my host at 6:40PDT, a five minute trip.
Unfortunately many mail readers hide or delete these headers upon delivery (ie Outlook 2000).