how to decode =?windows-874?B?t7TKzbogdGVzdCB=?= to Thai Langauge ?

Question

Dim TCP As Net.Sockets.TcpClient
Dim POP3Stream As System.IO.Stream
Dim inStream As System.IO.StreamReader

TCP = New Net.Sockets.TcpClient
TCP.Connect(strServer, 110)

POP3Stream = TCP.GetStream
inStream = New System.IO.StreamReader(POP3Stream, System.Text.Encoding.GetEncoding("utf-8"))

strTemp = inStream.ReadLine

While (strTemp <> ".")
strEmailMess = strEmailMess & strTemp & vbCrLf
strTemp = inStream.ReadLine
End While

For example: if strTemp is =?windows-874?B?t7TKzbogdGVzdCB=?=

i have to cut ?windows-874?B? and ?= so, it will remains t7TKzbogdGVzdCB=

and i will use code below to encode it to Thai letter

Dim encoding As New System.Text.UTF8Encoding
Dim Buffer As Byte() = Convert.FromBase64String(t7TKzbogdGVzdCB=)

DecodeBase64 = encoding.GetString(Buffer)

it's read ?????????

How to make it read in Thai letter ?

Any idea in C#?

Thanks