请问:为什么我这个SMTP邮件格式是纯文本呢?

Date: Sat, 26 Aug 2006 11:26:21 +437 (CST)
From: "j" <[email protected]>
To: "j" <[email protected]>
Subject: =?gb2312?B?YiBkaXI9IG5hbWU9YzpcbXlzZW5kLnR4dCBmaWxlcz0wIG51bT0x?=
Message-ID: <[email protected]>
X-Mailer: Nicend 1,1
Mime-Version: 1.0
Content-Type: multipart/mixed;
boundary="first"This is a multi-part message in MIME format.--first
Content-Type: multipart/alternative;
boundary="second"
--second
Content-Type: text/plain;
charset="gb2312"
Content-Transfer-Encoding: 7bitadfs--second
Content-Type: text/html;
charset="gb2312"
Content-Transfer-Encoding: 7bit<html><font color=red>fsa</font></html>--second--
--first
Content-Type: application/octet-stream;
name="mysend.txt"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
filename="mysend.txt"AS0NCg0KLg==--first--