用BASE64对邮件内容进行解码的问题

小弟目前在用WINPCAP做邮件的程序，邮件的内容都已经抓到了，但还要对邮件内容进行过滤，所以要用BASE64对邮件进行解码做关键字过滤，以下是邮件解码前的数据
--=====003_Dragon687870357613_=====
Content-Type: text/plain;
charset="gb2312"
Content-Transfer-Encoding: base64QmFzZTY0ysfSu9bWutyzo7z7tcSx4MLruea3tqOsxuTX99PDyse9q7b+vfjWxtDywdDXqru7zqrI
y8Dgv8m2wbXEQVNDSUnX1rf70PLB0KOss6PTw9Ta0OjTw82ouf3OxLG+0K3S6aOoscjI50hUVFC6
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yrnTw0Jhc2U2NMC0vNPD3L3iw9y1xMu1t6ijrLWr1eLA78v5y7W1xLzTw9zT673iw9zKtbzKysfW
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QmFzZTY0w/uzxrXE08nAtKGjDQoNCg0KDQoNCnRhbnl1MTk4Mw0KMjAwNy0wMy0yOQ0K--=====003_Dragon687870357613_=====
Content-Type: text/html;
charset="gb2312"
Content-Transfer-Encoding: base64PCFET0NUWVBFIEhUTUwgUFVCTElDICItLy9XM0MvL0RURCBIVE1MIDQuMCBUcmFuc2l0aW9uYWwv
L0VOIj4NCjxIVE1MPjxIRUFEPg0KPE1FVEEgaHR0cC1lcXVpdj1Db250ZW50LVR5cGUgY29udGVu
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TD4NCg==我应该怎么对它进行过滤呢？我想把中间的
--=====003_Dragon687870357613_=====
Content-Type: text/html;
charset="gb2312"
Content-Transfer-Encoding: base64
删掉，这样内容就连续了，解码就比较方便，解码的函数已经有了，关键是怎么判断才能把中间的那些内容删掉，谢谢各位大虾了