你好：
能把你的源代码贴出来供大家参考一下吗？
请参看一下这篇微软的官方文档：
http://support.microsoft.com/kb/2402064
好像在.net4.0应用程序中，使用 System.Net.SmtpClient 类的时候使用 Attachment 类来给邮件添加附件的时候，附件名称非ASCII编码并且长度超过41个UTF8字节的时候会出现这种问题。
按照微软的解决方案，你需要到以下链接中下载安装一个补丁包：
https://connect.microsoft.com/VisualStudio/Downloads/DownloadDetails.aspx?DownloadID=31723
请尝试一下这个解决方案，更多内容请仔细阅读上面的微软文档。
Caillen
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Thanks
MSDN Community Support
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