Imports System.ComponentModel
Imports System.Runtime.InteropServices

    '***************************************
    ' Private Global Properties/Variables/Objects
    '***************************************
    Private Enum QueryOptions
        DNS_QUERY_STANDARD = &H0
        DNS_QUERY_ACCEPT_TRUNCATED_RESPONSE = &H1
        DNS_QUERY_USE_TCP_ONLY = &H2
        DNS_QUERY_NO_RECURSION = &H4
        DNS_QUERY_BYPASS_CACHE = 8
        DNS_QUERY_NO_WIRE_QUERY = &H10
        DNS_QUERY_NO_LOCAL_NAME = &H20
        DNS_QUERY_NO_HOSTS_FILE = &H40
        DNS_QUERY_NO_NETBT = &H80
        DNS_QUERY_TREAT_AS_FQDN = &H1000
        DNS_QUERY_WIRE_ONLY = &H100
        DNS_QUERY_RETURN_MESSAGE = &H200
        DNS_QUERY_DONT_RESET_TTL_VALUES = &H100000
        DNS_QUERY_RESERVED = &HFF000000 '-16777216
    End Enum
    Private Enum QueryTypes
        DNS_TYPE_A = 1
        DNS_TYPE_NS = 2
        DNS_TYPE_MD = 3
        DNS_TYPE_MF = 4
        DNS_TYPE_CNAME = 5
        DNS_TYPE_SOA = 6
        DNS_TYPE_MB = 7
        DNS_TYPE_MG = 8
        DNS_TYPE_MR = 9
        DNS_TYPE_NULL = 10
        DNS_TYPE_WKS = 11
        DNS_TYPE_PTR = 12
        DNS_TYPE_HINFO = 13
        DNS_TYPE_MINFO = 14
        DNS_TYPE_MX = 15
        DNS_TYPE_TEXT = 16
    End Enum
    Private Structure MXRecord
        Dim ptrNext As IntPtr
        Dim strName As String
        Dim intType As Short
        Dim intDataLength As Short
        Dim intFlags As Integer
        Dim intTTL As Integer
        Dim intReserved As Integer
        Dim ptrNameExchange As IntPtr
        Dim intPreference As Short
        Dim intPad As Short
    End Structure

    '***************************************
    ' API Declarations
    '***************************************
    Private Declare Auto Function DNSQuery Lib "DNSAPI.dll" Alias "DnsQuery_W" (ByVal Name As String, ByVal Type As QueryTypes, ByVal Options As QueryOptions, ByVal Servers As Integer, ByVal QueryResults As IntPtr, ByVal Reserved As Integer) As Integer
    Private Declare Auto Sub DNSRecordListFree Lib "DNSAPI.dll" Alias "DnsRecordListFree" (ByVal RecordList As IntPtr, ByVal FreeType As Integer)

    Private Function GetMXRecord(ByVal Domain As String) As String()
        '************************************************************************
        ' Procedure/Function: GetMXRecord()
        ' Author: Ben Santiago
        ' Created On: 12/06/2006
        ' Description:
        '       Retrieves list of MX Record from default DNS Server for specified domain.
        '************************************************************************

        '***************************************
        ' Check OS Version
        '***************************************
        If Not (Environment.OSVersion.Platform = PlatformID.Win32NT) Then
            Throw New NotSupportedException
        End If

        '***************************************
        ' Initialize Variables
        '***************************************
        Dim strServerName As String = ""
        Dim ptrFirstItemInList As IntPtr = IntPtr.Zero
        Dim ptrCurrentItemInList As IntPtr = IntPtr.Zero
        Dim objMXRecord As MXRecord
        Dim arrReturnList As New ArrayList
        Dim intReturnValue As Integer = DNSQuery(Domain, QueryTypes.DNS_TYPE_MX, QueryOptions.DNS_QUERY_BYPASS_CACHE, Nothing, ptrFirstItemInList, 0)


        '***************************************
        ' Throw Exception If Error
        '***************************************
        If intReturnValue <> 0 Then
            Throw New Win32Exception(intReturnValue)
        End If

        '***************************************
        ' Process Each Returned MX Record
        '***************************************
        ptrCurrentItemInList = ptrFirstItemInList
        While Not ptrCurrentItemInList.Equals(IntPtr.Zero)
            objMXRecord = CType(Marshal.PtrToStructure(ptrCurrentItemInList, GetType(MXRecord)), MXRecord)
            If objMXRecord.intType = 15 Then
                strServerName = Marshal.PtrToStringAuto(objMXRecord.ptrNameExchange)
                arrReturnList.Add(strServerName)
            End If
            ptrCurrentItemInList = objMXRecord.ptrNext
        End While

        '***************************************
        ' Release DNS Record List and Return String Array
        '***************************************
        DNSRecordListFree(ptrCurrentItemInList, 0)
        Return CType(arrReturnList.ToArray(GetType(String)), String())
    End Function