Exchange 2007 Outgoing mail problem.

I have a domain (SBS 2008) with Exchange 2007.
I have users that have domain email accounts and email accounts that have a separate domain name, hosted by an ISP. In other words, two separate email accounts. Three users are accessing their email from an offsite location using Outlook Web Access. One user (User 4) is accessing his email from within the domain using MS Outlook 2007.

User1@SBS_Domain.com  & User1@ISP_Domain.com  
User2@SBS_Domain.com & User1@ISP_Domain.com
User3@SBS_Domain.com & User1@ISP_Domain.com
User4@SBS_Domain.com & User1@ISP_Domain.com

The problem:
Once upon a time, everyone was working from within the SBS Domain. Until one day, users 2 & 3 started receiving tremendous amounts of spam. 1 &2 moved to a new physical location, along with user #3. I created new ISP_domain email accounts for users 1 & 2 (ex: User.1@ISP_Domain.com / User.2@ISP_Domain.com), because the spam situation had been out of control.
Users 1, 2 &3 are using Outlook web access to get their mail. User 3 is not having any issues. Users 1 & 2 are not able to receive email from User 4(within SBS domain), but everything if fine from the rest of the world, that is outside of the SBS domain.

Here is a copy of the NDR User 4 receives when trying to send a message to User 1.

Delivery has failed to these recipients or distribution lists:
User1@ISPdomain.com
The recipient's e-mail address was not found in the recipient's e-mail system. Microsoft Exchange will not try to redeliver this message for you. Please check the e-mail address and try resending this message, or provide the following diagnostic text to your system administrator
Sent by Microsoft Exchange Server 2007
Diagnostic information for administrators:
Generating server: SBS_Domain.COM
User1@ISPdomain.com
#550 5.1.1 RESOLVER.ADR.RecipNotFound; not found ##
Original message headers:

Received: fromSBS_Domain.COM([fe80::3842:f81e:b98:ffb4]) by
SBS_Domain.COM([fe80::3842:f81e:b98:ffb4%10]) with mapi; Tue, 1 Nov
 2011 12:26:50 -0800
Content-Type: application/ms-tnef; name="winmail.dat"
Content-Transfer-Encoding: binary
From: User4 <User4@SBSDomain.com
To: User1 <User1@ISPdomain.com>
Date: Tue, 1 Nov 2011 12:26:47 -0800
Subject: 11/1/11 Test #1
Thread-Topic: 11/1/11 Test #1
Thread-Index: AcyY1JR47ug1+T8QRWKr6nJx/TnU0A==
Message-ID: <558C49C134BA9340B71221CC29728ED1315382D81C@MLA03.MAPPINGALASKA.COM>
Accept-Language: en-US
Content-Language: en-US
X-MS-Has-Attach:
X-MS-TNEF-Correlator: <558C49C134BA9340B71221CC29728ED1315382D81C@MLA03.MAPPINGALASKA.COM>
x-cr-puzzleid: {90400334-3F62-46BB-9F35-9CB9A1ED47EA}
x-cr-hashedpuzzle: AhK/ BK3n CHo+ C6x3 C8bf Dpiq ENOb FVWm Ff3v HMaY H2wT
 IwTr I7/0 JmW2 J8oc
 Lew1;1;awBlAHYAaQBuAC4AcwBpAGwAdgBlAHIAbgBhAGwAZQBAAGcAcABzAGEAbABhAHMAawBhAC4AYwBvAG0A;Sosha1_v1;7;{90400334-3F62-46BB-9F35-9CB9A1ED47EA};awBzAHQAZQB2AGUAbgBzAEAAbQBhAHAAcABpAG4AZwBhAGwAYQBzAGsAYQAuAGMAbwBtAA==;Tue,
 01 Nov 2011 20:26:47 GMT;MQAxAC8AMQAvADEAMQAgAFQAZQBzAHQAIAAjADEA
MIME-Version: 1.0