1. System.Web.Mail - хотя и помечен в документации как устаревший - шлет такие заголовки:
From test@------------------- Fri May 12 15:46:44 2006
Return-path: <test@------------------->
Envelope-to: andres@-------------------
Delivery-date: Fri, 12 May 2006 15:46:44 +0300
Received: from [194.126.101.98] (helo=mail.neti.ee)
by zeus.serverglobe.net with esmtp (Exim 4.52)
id 1FeX2Y-0008KV-RK
for andres@-------------------; Fri, 12 May 2006 15:46:43 +0300
Received: from pooh (80-235-75-230-dsl.medium.estpak.ee [80.235.75.230])
by Relayhost1.neti.ee (Postfix) with SMTP id 2B39318273
for <andres@------------------->; Fri, 12 May 2006 15:46:40 +0300 (EEST)
thread-index: AcZ1why0IEZcWMPsQsKZn+6hHpMf4g==
Thread-Topic: Test
From: <test@------------------->
To: <andres@------------------->
Subject: Test
Date: Fri, 12 May 2006 15:46:39 +0300
Message-ID: <000001c675c2$1cbbc210$6400a8c0@pooh>
MIME-Version: 1.0
Content-Type: text/plain;
charset="windows-1257"
Content-Transfer-Encoding: 7bit
X-Mailer: Microsoft CDO for Windows 2000
Content-Class: urn:content-classes:message
Importance: normal
Priority: normal
X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.2869
X-Virus-Scanned: by amavisd-new-2.2.1 (20041222) (Debian) at neti.ee
X-Antivirus-Scanner: Clean mail though you should still use an Antivirus
2. Можно попробовать этот класс
http://www.codeproject.com/csharp/smtpemailer.aspТо, что лежит на codeproject.com можно использовать свободно, проблем с лицензированием нет.