POP 服务器允许使用 LIST 命令返回邮箱中所有电子邮件的列表。不幸的是，它不会返回所有电子邮件，它只返回收件箱中的电子邮件。因此，如果电子邮件落入垃圾文件夹，它就找不到它。
是否可以使用 POP 从垃圾文件夹中下载电子邮件？
这是我当前使用的类：
Option Strict On
Option Explicit On
Imports System.Net, System.Text
Public Class POP3
Inherits Sockets.TcpClient
Dim Stream As Sockets.NetworkStream
Dim UsesSSL As Boolean = False
Dim SslStream As Security.SslStream
Dim SslStreamDisposed As Boolean = False
Public LastLineRead As String = vbNullString
Public Overloads Sub Connect(ByVal Server As String, ByVal Username As String, ByVal Password As String, Optional ByVal InPort As Integer = 110,Optional ByVal UseSSL As Boolean = False)
If Connected Then Disconnect()
UsesSSL = UseSSL
MyBase.Connect(Server, InPort)
Stream = MyBase.GetStream
If UsesSSL Then
SslStream = New Security.SslStream(Stream)
SslStream.AuthenticateAsClient(Server)
End If
If Not CheckResponse() Then Exit Sub
If CBool(Len(Username)) Then
Me.Submit("USER " & Username & vbCrLf)
If Not CheckResponse() Then Exit Sub
End If
If CBool(Len(Password)) Then
Me.Submit("PASS " & Password & vbCrLf)
If Not CheckResponse() Then Exit Sub
End If
End Sub
Public Function CheckResponse() As Boolean
If Not IsConnected() Then Return False
LastLineRead = Me.Response
If (Left(LastLineRead, 3) <> "+OK") Then
Throw New POP3Exception(LastLineRead)
Return False
End If
Return True
End Function
Public Function IsConnected() As Boolean
If Not Connected Then
Throw New POP3Exception("Not Connected to an POP3 Server.")
Return False
End If
Return True
End Function
Public Function Response(Optional ByVal dataSize As Integer = 1) As String
Dim enc As New ASCIIEncoding
Dim ServerBufr() As Byte
Dim Index As Integer = 0
If dataSize > 1 Then
ReDim ServerBufr(dataSize - 1)
Dim dtsz As Integer = dataSize
Dim sz As Integer
Do While Index < dataSize
If UsesSSL Then
sz = SslStream.Read(ServerBufr, Index, dtsz)
Else
sz = Stream.Read(ServerBufr, Index, dtsz)
End If
If sz = 0 Then Return vbNullString
Index += sz
dtsz -= sz
Loop
Else
ReDim ServerBufr(255)
Do
If UsesSSL Then
ServerBufr(Index) = CByte(SslStream.ReadByte)
Else
ServerBufr(Index) = CByte(Stream.ReadByte)
End If
If ServerBufr(Index) = -1 Then Exit Do
Index += 1
If ServerBufr(Index - 1) = 10 Then Exit Do
If Index > UBound(ServerBufr) Then
ReDim Preserve ServerBufr(Index + 255)
End If
Loop
End If
Return enc.GetString(ServerBufr, 0, Index)
End Function
Public Sub Submit(ByVal message As String)
Dim enc As New ASCIIEncoding
Dim WriteBuffer() As Byte = enc.GetBytes(message)
If UsesSSL Then
SslStream.Write(WriteBuffer, 0, WriteBuffer.Length)
Else
Stream.Write(WriteBuffer, 0, WriteBuffer.Length)
End If
End Sub
Public Sub Disconnect()
Me.Submit("QUIT" & vbCrLf)
CheckResponse()
If UsesSSL Then
SslStream.Dispose()
SslStreamDisposed = True
End If
End Sub
'*******************************************************************************
' Function Name : List
' Purpose : Get the drop listing from the maildrop
' :
' Returns : Any Arraylist of POP3Message objects
' :
' Typical telNet I/O:
'LIST (submit)
'+OK Mailbox scan listing follows
'1 2532 (record index and size in bytes)
'2 1610
'3 12345
'. (end of records terminator)
'*******************************************************************************
Public Function List() As ArrayList
If Not IsConnected() Then Return Nothing 'exit if not in TRANSACTION mode
Me.Submit("LIST" & vbCrLf) 'submit List request
If Not CheckResponse() Then Return Nothing 'check for a response, but if an error, return nothing
'
'get a list of emails waiting on the server for the authenticated user
'
Dim retval As New ArrayList 'set aside message list storage
Do
Dim response As String = Me.Response 'check response
If (response = "." & vbCrLf) Then 'done with list?
Exit Do 'yes
End If
Dim msg As New POP3Message 'establish a new message
Dim msgInfo() As String = Split(response, " "c) 'separate by spaces, which divide its fields
msg.MailID = Integer.Parse(msgInfo(0)) 'get the list item number
msg.ByteCount = Integer.Parse(msgInfo(1)) 'get the size of the email message
msg.Retrieved = False 'indicate its message body is not yet retreived
retval.Add(msg) 'add a new entry into the retrieval list
Loop
Return retval 'return the list
End Function
Public Function GetHeader(ByRef msg As POP3Message, Optional ByVal BodyLines As Integer = 0) As POP3Message
If Not IsConnected() Then Return Nothing
Me.Submit("TOP " & msg.MailID.ToString & " " & BodyLines.ToString & vbCrLf)
If Not CheckResponse() Then Return Nothing
msg.Message = vbNullString
Do
Dim response As String = Me.Response
If response = "." & vbCrLf Then
Exit Do
End If
msg.Message &= response
Loop
Return msg
End Function
Public Function Retrieve(ByRef msg As POP3Message) As POP3Message
If Not IsConnected() Then Return Nothing
Me.Submit("RETR " & msg.MailID.ToString & vbCrLf)
If Not CheckResponse() Then Return Nothing
msg.Message = Me.Response(msg.ByteCount)
Do
Dim S As String = Response()
If S = "." & vbCrLf Then
Exit Do
End If
msg.Message &= S
Loop
msg.ByteCount = Len(msg.Message)
Return msg
End Function
Public Sub Delete(ByVal msgHdr As POP3Message)
If Not IsConnected() Then Exit Sub
Me.Submit("DELE " & msgHdr.MailID.ToString & vbCrLf)
CheckResponse()
End Sub
Public Sub Reset()
If Not IsConnected() Then Exit Sub
Me.Submit("RSET" & vbCrLf)
CheckResponse()
End Sub
Public Function NOOP() As Boolean
If Not IsConnected() Then Return False
Me.Submit("NOOP")
Return CheckResponse()
End Function
Protected Overrides Sub Finalize()
If Not SslStreamDisposed Then
SslStream.Dispose()
End If
MyBase.Finalize()
End Sub
End Class
Public Class POP3Message
Public MailID As Integer = 0
Public ByteCount As Integer = 0
Public Retrieved As Boolean = False
Public Message As String = vbNullString
Public Overrides Function ToString() As String
Return Message
End Function
End Class
Public Class POP3Exception
Inherits ApplicationException
Public Sub New(ByVal str As String)
MyBase.New(str)
End Sub
End Class
根据评论，POP3 标准只允许从“收件箱”下载。它不是为更高级的东西而设计的。
理想的解决方案是使用 IMAP4（如果邮件服务器支持）。
IMAP4 允许您保存、标记、复制和删除邮件，以及允许文件夹和子文件夹，并且不需要独占访问权限。
我们也遇到同样的问题，正在寻找不会将电子邮件路由到服务器垃圾邮件文件夹的电子邮件提供商。我们会使用 IMAP，但在某些计算机上使用 IMAP 时，仅发送和接收就需要半个小时。随着计算机变得越来越强大，它们变得越来越慢。