ASP.NET

public static bool IsValidEmailDomain(MailAddress address)
{
    if (address == null) return false;
    
    // TODO: Check what this method does with an invalid or unknown domain.
    // If it throws an exception, you'll need a try..catch block.
    var response = DnsClient.Default.Resolve(address.Host, RecordType.Mx);
    if (response == null || response.AnswerRecords == null) return false;
    
    return response.AnswerRecords.OfType<MxRecord>().Any();
}

public static bool IsValidEmailDomain(string address)
{
    if (string.IsNullOrWhiteSpace(address)) return false;
    
    MailAddress theAddress;
    try
    {
        theAddress = new MailAddress(address);
    }
    catch (FormatException)
    {
        return false;
    }
    
    return IsValidEmailDomain(theAddress);
}

public static bool IsValidEmailDomain(MailAddress address)
{
    if (address == null) return false;
    var response = DnsClient.Default.Resolve(address.Host, RecordType.Mx);
    try
    {
        if (response == null || response.AnswerRecords == null) return false;
    }
    catch (FormatException)
    {
        return false;
    }

    return response.AnswerRecords.OfType<MxRecord>().Any();
}

public static string test()
   {
       string mail = "#####6@gmail.com";
       bool? answer = null;
       Exception ex;
       try
       {
           answer = IsValidEmailDomain(mail);
       }
       catch (Exception e)
       {
           ex = e;
       }
       if (answer.HasValue)
       {
           return answer.ToString();
       }
       else
       {
           return null;
           // catch exception
       }
   }

public static string mailMX()
   {
       string domainName = "#######.com";
       string mail = "";

       var response = DnsClient.Default.Resolve(domainName, RecordType.Mx);
       var records = response.AnswerRecords.OfType<MxRecord>();
      foreach (var record in records)
       {
          return (record.ExchangeDomainName).ToString();
       }

      return mail = records.ToString();
   }

public static bool IsValidEmailDomain(MailAddress address)
{
    if (address == null) return false;

    var response = DnsClient.Default.Resolve(address.Host, RecordType.Mx);

    if (response == null) return false;
    if (response.ReturnCode != ReturnCode.NoError && response.ReturnCode != ReturnCode.NxDomain) return false;
    if (response.AnswerRecords == null) return false;

    return response.AnswerRecords.OfType<MxRecord>().Any();
}

public static bool IsValidEmailDomain(string address)
{
    if (string.IsNullOrWhiteSpace(address)) return false;

    MailAddress theAddress;
    try
    {
        theAddress = new MailAddress(address);
    }
    catch (FormatException)
    {
        return false;
    }

    return IsValidEmailDomain(theAddress);
}

IsValidEmailDomain("test@gmail.com") == true;
IsValidEmailDomain("test@test.test") == false;

nslookup
set type=mx
gmail.com