求助base64解码问题

delphi xe7 update1+indy10.6收邮件，有一种邮件标题和正文含有汉字就乱码。将IdMessage1的NoDecode := True;和NoEncode := True;保存为.eml文件，用Foxmail打开正文则汉字显示正常。
用自带的base64解码函数DecodeString(IdMessage1.Body.Text);则报错：
.eml文件部分内容如下：
Subject: ??????????????360??????????????????????????????????
X-PHP-Originating-Script: 1017:func.php
MIME-Version: 1.0rn
Content-type: text/html
From: 360???????????? <[email protected]>
Content-Transfer-Encoding: base64
X-MIME-Autoconverted: from 8bit to base64 by se4.white.zwt.qihoo.net id t5H7Xtwt015350PCFET0NUWVBFIGh0bWwgUFVCTElDICItLy9XM0MvL0RURCBYSFRNTCAxLjAgVHJhbnNpdGlv
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