Internal Exchange Users keep getting un-deliverable messages to removed internal users...

Here is the skinny...

I am came to my current company several months ago and have been bombarded with Exchange issues since I started.  I have several users that want to be off exchange and I am trying to accommodate their requests.  Here is how we are setup (I did not do it this way, the old Sys. Admin and the owner did):

• We use Gmail for our domain emails: user@company.com
• For internal users we forward their Gmail account to our internal exchange server: user@exchange.company.com
• The internal users have AD accounts and are set on the local domain: company.local

When trying to remove user from Exchange the following steps are taken:

• Disable user account in EMC
• Update GAL and OAB
• Turn off auto forwarding in users Gmail account

When using another internal users Outlook, the following is done to update outlook:

• updates show in other internal users address books after force update from Outlook (File>Account Settings>Update Address Book
• Start typing removed users Gmail address, click delete when comes up in auto-complete, and finish typing address (address does not resolve to a name and stays the full address: user@company.com)
• Click send

Then the following occurs:

• Email NDR is received as follows:

Received: from exchange.company.local ([127.0.0.1]) by exchange ([127.0.0.1])
 with mapi; Mon, 25 Nov 2013 08:09:46 -0700
Content-Type: application/ms-tnef; name="winmail.dat"
Content-Transfer-Encoding: binary
From: First Last <first.last@company.com>
To: First Last
        <IMCEAEX-_O=555+20OFFICE+20ORGANIZATION_OU=EXCHANGE+20ADMINISTRATIVE+20GROUP+20+28FYDIBOHF23SPDLT+29_CN=RECIPIENTS_CN=First+2Elast@company.local>
Date: Mon, 25 Nov 2013 08:09:44 -0700
Subject: Test
Thread-Topic: Test
Thread-Index: Ac7p8F+hsJRQvn3KT1uRPoqLwuQEOw==
Message-ID: <5C3E4E1C623D1A4682962420D58CE993289FF776@xchange>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach:
X-MS-TNEF-Correlator: <5C3E4E1C623D1A4682962420D58CE993289FF776@xchange>
MIME-Version: 1.0

I have done the following to try and remedy this issue:

• Used ADSI Edit to remove any instance of user@company.local from users AD account
• removed user from auto-complete file
• removed user from auto-complete file using NK2Edit tool
• changed the IMCEAEX in ADSI Edit to be an X500 address
• I have not completely deleted the auto-complete file as many of our internal users have hundreds of addresses in their files.

The main problem is that any internal user using Exchange still gets a NDR after updating all address books.  Somewhere, the Exchange or AD server are still showing the removed user as internal.  However, logging into OWA proves to be successful in sending to the removed users Gmail account with no issues.  Also, any external emails will go to the users Gmail account as well.  

This problem is only coming from other internal users.  I need to find a way to update Exchange to fix this issue so that we can get these users off of Exchange.  The ultimate goal is to get rid of Exchange completely and only use Outlook with Google Business Apps.  (This is not my idea, but the owner insists on doing this...It is my job to make it work)

Thanks!!