// Here is the Test Console application that is designed to send an email with very large attachment to the Email Service:

                EmailSerClient proxy = new EmailSerClient();
                EmailMessage emailMsg = new EmailMessage();
                emailMsg.EmailParamHeaders = new DataContracts.Email();

                emailMsg.EmailParamHeaders.To = new string[2];
                emailMsg.EmailParamHeaders.CC = new string[1];

                emailMsg.EmailParamHeaders.From = "noreply@xys.com";
                emailMsg.EmailParamHeaders.To[0] = "abc@xys.com";

                emailMsg.EmailParamHeaders.Subject = "Test line here";
                emailMsg.EmailParamHeaders.Message = "test message here.";
                emailMsg.EmailParamHeaders.AttachmentFileName = @"c:\log\attchment.zip";  //null; 

                string filePath = System.IO.Path.Combine(@"c:\log\", "attachment.zip");
                System.IO.FileInfo fileInfo = new System.IO.FileInfo(filePath);

               //  open stream
                System.IO.FileStream stream = new System.IO.FileStream(@"c:\log\attachment.zip",
                System.IO.FileMode.Open, System.IO.FileAccess.Read);

               // See RemoteFileInfo class below:
                RemoteFileInfo result = new RemoteFileInfo();

                // return result 
                result.FileName = "attachment.zip";
                result.Length = fileInfo.Length;
                result.FileByteStream = stream;

                try
                {
                    proxy.SendEmail(emailMsg.EmailParamHeaders, result.FileByteStream);//Stream.Null);

                }
                catch (Exception e)
                {
                }
       
            }

// Here is the RemoteFileInfo class which I think might be part of the problem:

using System.Linq;
using System.Text;
using System.Runtime.Serialization;
using System.IO;
using System.ServiceModel.Web;
using System.ServiceModel;

namespace wcfTestConsoleApp
{
    [MessageContract]
    public class RemoteFileInfo : IDisposable
    {
        [MessageHeader(MustUnderstand = true)]
        public string FileName;

        [MessageHeader(MustUnderstand = true)]
        public long Length;

        [MessageBodyMember(Order = 1)]
        public System.IO.Stream FileByteStream;

        public void Dispose()
        {
            if (FileByteStream != null)
            {
                FileByteStream.Close();
                FileByteStream = null;
            }
        }
    }
}

// Here is the config file for the Test Client above:

<configuration>
    <system.serviceModel>
        <bindings>
            <basicHttpBinding>
                <binding name="BasicHttpBinding_IEmailService" closeTimeout="00:01:00" openTimeout="00:01:00" receiveTimeout="00:10:00" sendTimeout="00:01:00" bypassProxyOnLocal="false" hostNameComparisonMode="StrongWildcard" maxBufferPoolSize="50000000" maxReceivedMessageSize="50000000" messageEncoding="Text" textEncoding="utf-8" useDefaultWebProxy="true" allowCookies="false">
                    <readerQuotas maxDepth="32" maxStringContentLength="50000000" maxArrayLength="50000000" maxBytesPerRead="50000000" maxNameTableCharCount="50000000" />

                    <security mode="Message">
                        <transport clientCredentialType="Windows" proxyCredentialType="None" realm="" />
                        <message algorithmSuite="Default" />
                    </security>
                </binding>
            </basicHttpBinding>
        </bindings>
        <client>
            <endpoint address="http://localhost/EmailWcfService/EmailService.svc"
                binding="basicHttpBinding" bindingConfiguration=""
                contract="IEmailService" name="BasicHttpBinding_IEmailService">
                <identity>
                    <servicePrincipalName value="host/abcdefg.com" />
                </identity>
            </endpoint>
        </client>
    </system.serviceModel>
</configuration>

// Here below is the Email service's config file in relation to serviceModel/binding...etc.:

<pre lang="xml"><system.serviceModel>
      <behaviors>
          <serviceBehaviors>
              <behavior name="EmailWCFServiceTypeBehaviors">
                  <serviceMetadata httpGetEnabled="true"/>
                 
                  <serviceDebug includeExceptionDetailInFaults="true"/>
              </behavior>
          </serviceBehaviors>
      </behaviors>
      <bindings>
          <basicHttpBinding>
              <binding name="httpLargeMessageStream"         transferMode="StreamedRequest" maxReceivedMessageSize="52428800" maxBufferSize="52428800"/>
          </basicHttpBinding>
      </bindings>
      <services>
    <service name="EmailService" behaviorConfiguration="EmailWCFServiceTypeBehaviors">
              
      <endpoint address="" binding="basicHttpBinding" contract="IEmailService"  bindingNamespace=""/>

      <serviceHostingEnvironment multipleSiteBindingsEnabled="true"/>
</system.serviceModel>

<system.webServer>
     <modules runAllManagedModulesForAllRequests="true"/>
     <directoryBrowse enabled="true"/>
</system.webServer