http://www.eggheadcafe.com/articles/20050129.asp

by Peter A. Bromberg, Ph.D.

namespace PAB.DnsUtils
{
using System;
using System.Collections;
using System.ComponentModel;
using System.Runtime.InteropServices;

public class DnsMx
{
public DnsMx()
{
}
[DllImport("dnsapi", EntryPoint="DnsQuery_W", CharSet=CharSet.Unicode, SetLastError=true, ExactSpelling=true)]
private static extern int DnsQuery([MarshalAs(UnmanagedType.VBByRefStr)]ref string pszName, QueryTypes wType, QueryOptions options, int aipServers, ref IntPtr ppQueryResults, int pReserved);

[DllImport("dnsapi", CharSet=CharSet.Auto, SetLastError=true)]
private static extern void DnsRecordListFree(IntPtr pRecordList, int FreeType);

public static string[] GetMXRecords(string domain)
{

IntPtr ptr1=IntPtr.Zero ;
IntPtr ptr2=IntPtr.Zero ;
MXRecord recMx;
if (Environment.OSVersion.Platform != PlatformID.Win32NT)
{
throw new NotSupportedException();
}
ArrayList list1 = new ArrayList();
int num1 = DnsMx.DnsQuery(ref domain, QueryTypes.DNS_TYPE_MX, QueryOptions.DNS_QUERY_BYPASS_CACHE, 0, ref ptr1, 0);
if (num1 != 0)
{
throw new Win32Exception(num1);
}
for (ptr2 = ptr1; !ptr2.Equals(IntPtr.Zero); ptr2 = recMx.pNext)
{
recMx = ( MXRecord) Marshal.PtrToStructure(ptr2, typeof(MXRecord));
if (recMx.wType == 15)
{
string text1 = Marshal.PtrToStringAuto(recMx.pNameExchange);
list1.Add(text1);
}
}
DnsMx.DnsRecordListFree(ptr2, 0);
return (string[]) list1.ToArray(typeof(string));
}

private enum QueryOptions
{
DNS_QUERY_ACCEPT_TRUNCATED_RESPONSE = 1,
DNS_QUERY_BYPASS_CACHE = 8,
DNS_QUERY_DONT_RESET_TTL_VALUES = 0x100000,
DNS_QUERY_NO_HOSTS_FILE = 0x40,
DNS_QUERY_NO_LOCAL_NAME = 0x20,
DNS_QUERY_NO_NETBT = 0x80,
DNS_QUERY_NO_RECURSION = 4,
DNS_QUERY_NO_WIRE_QUERY = 0x10,
DNS_QUERY_RESERVED = -16777216,
DNS_QUERY_RETURN_MESSAGE = 0x200,
DNS_QUERY_STANDARD = 0,
DNS_QUERY_TREAT_AS_FQDN = 0x1000,
DNS_QUERY_USE_TCP_ONLY = 2,
DNS_QUERY_WIRE_ONLY = 0x100
}

private enum QueryTypes
{
DNS_TYPE_MX = 15
}

[StructLayout(LayoutKind.Sequential)]
private struct MXRecord
{
public IntPtr pNext;
public string pName;
public short wType;
public short wDataLength;
public int flags;
public int dwTtl;
public int dwReserved;
public IntPtr pNameExchange;
public short wPreference;
public short Pad;
}
}
}

static void Main(string[] args)
{
string[] s= PAB.DnsUtils.DnsMx.GetMXRecords("microsoft.com");
foreach (string st in s)
Console.WriteLine("Server: {0}",st);
Console.ReadLine();