GetMXRecords in C#

MailUtil.cs

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Linq;
using System.Runtime.InteropServices;
using System.Diagnostics;

public class MailUtil
{
    #region DLL Imports
    [DllImport("dnsapi", CharSet = CharSet.Unicode, SetLastError = true, ExactSpelling = true)]
    static extern int DnsQuery_W([MarshalAs(UnmanagedType.LPWStr)]string pszName, DnsRecordType wType, QueryOptions options, int aipServers, ref IntPtr ppQueryResults, int pReserved);

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

    enum QueryOptions
    {
        DNS_QUERY_STANDARD = 0,
        DNS_QUERY_ACCEPT_TRUNCATED_RESPONSE = 1,
        DNS_QUERY_USE_TCP_ONLY = 2,
        DNS_QUERY_NO_RECURSION = 4,
        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_WIRE_QUERY = 0x10,
        DNS_QUERY_RESERVED = -16777216,
        DNS_QUERY_RETURN_MESSAGE = 0x200,
        DNS_QUERY_TREAT_AS_FQDN = 0x1000,
        DNS_QUERY_WIRE_ONLY = 0x100
    }

    enum DnsRecordType : short
    {
        DNS_TYPE_A = 1,
        DNS_TYPE_NS = 2,
        DNS_TYPE_CNAME = 5,
        DNS_TYPE_SOA = 6,
        DNS_TYPE_PTR = 12,
        DNS_TYPE_HINFO = 13,
        DNS_TYPE_MX = 15,
        DNS_TYPE_TXT = 16,
        DNS_TYPE_AAAA = 28
    }

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

    public static List<string> GetMXRecords(string domain)
    {
        IntPtr ppResults = IntPtr.Zero;
        MXRecord mxRecord;

        var results = new List<Tuple<short, string>>();
        int error = DnsQuery_W(domain, DnsRecordType.DNS_TYPE_MX, QueryOptions.DNS_QUERY_STANDARD, 0, ref ppResults, 0);
        if (error != 0)
            throw new Win32Exception(error);

        for (var pRecord = ppResults; pRecord != IntPtr.Zero; pRecord = mxRecord.pNext)
        {
            mxRecord = (MXRecord)Marshal.PtrToStructure(pRecord, typeof(MXRecord));
            if (mxRecord.wType == DnsRecordType.DNS_TYPE_MX)
            {
                string name = Marshal.PtrToStringAuto(mxRecord.pNameExchange);
                results.Add(new Tuple<short, string>(mxRecord.wPreference, name));
            }
        }

        DnsRecordListFree(ppResults, 0);

        return results
            .OrderBy(x => x.Item1)
            .Select(x => x.Item2)
            .ToList();
    }

    static void Test()
    {
        var domains = "gmail.com yahoo.com hotmail.com".Split(' ');
        foreach (string domain in domains)
            foreach (var mx in GetMXRecords(domain))
                Debug.WriteLine(mx);
    }

}