C# POP3接收邮件 怎样将正文解码呢？

"This is a multi-part message in MIME format.\r\n\r\n------=_NextPart_5530AD57_08A2BB38_2683E8FB\r\nContent-Type: text/plain;\r\n\tcharset=\"gb18030\"\r\nContent-Transfer-Encoding: base64\r\n\r\nZGRkZGRkZGRkZGQ=\r\n\r\n------=_NextPart_5530AD57_08A2BB38_2683E8FB\r\nContent-Type: text/html;\r\n\tcharset=\"gb18030\"\r\nContent-Transfer-Encoding: base64\r\n\r\nPGRpdj5kZGRkZGRkZGRkZDwvZGl2Pg==\r\n\r\n------=_NextPart_5530AD57_08A2BB38_2683E8FB--\r\n\r\n"

http://blog.csdn.net/chinacsharper/article/details/21749763

正文其实就是base64编码，直接解码即可。当然，对于完整的邮件，用一个现成的库比较简单。
http://www.codeproject.com/Articles/19777/MIME-Compliant-Parser
http://www.codeproject.com/Articles/11882/Advanced-MIME-Parser-Creator-Editor
http://www.codeproject.com/Articles/11380/A-C-Implementation-of-Mime-De-encode

这个第一格怎么填？