编码转换参考

Dim file As String = "文件.txt"
Dim bytes() As Byte
Dim utf As Encoding = Encoding.GetEncoding("utf-8")  
Dim def As Encoding = Encoding.Default
Bytes = def.GetBytes(file)
Bytes = Encoding.Convert(def, utf, Bytes)  
file = def.GetString(Bytes)  
Output.Show(file)

----------------------

Dim file As String = "鏂囦欢.txt"
Dim bytes() As Byte
Dim utf As Encoding = Encoding.GetEncoding("utf-8")  
Dim def As Encoding = Encoding.Default
Bytes = utf.GetBytes(file)
Bytes = Encoding.Convert(utf, def, Bytes)
file = utf.GetString(Bytes)  
Output.Show(file)

比如

Dim file As String = "鏂囦欢.txt"
Dim bytes() As Byte
Bytes = Encoding.Default.GetBytes(file)
file = Encoding.utf8.GetString(Bytes)  
Output.Show(file)

...

进去后, 第一个窗口, 下面有一个 收取邮件  的按钮

下载信息  [文件大小：   下载次数： ]

点击浏览该文件:楽天市場-発送情報更新.rar

转换一下，如

Dim str As String = msg.MessageBody(msg.MessageBody.Count - 1)
Dim bytes() As Byte
Bytes = Encoding.Default.GetBytes(str)
str = Encoding.utf8.GetString(Bytes)

output.show(str)

但是，还是会有部分乱码，应噶是dll本身有问题的。