Legacy app can't send emails through Exchange 2010 - Send / Recevie connectors

Hi,

We have a legacy application that is installed on a bunch of workstations. The application uses a .dat file that is placed on a shared drive and read the SMTP configuration (email account, server IP, port, etc.)

We are ready to switch the email flow from Exchange 2003 to Exchange 2010 but I need the application to send through the new server first.

When sending through the CASArray IP the email only goes through when is being sent to an internal email address, however any attempt to send to an external domain it fails.

My connectors are configured as follows:

1.) Send Connector:

a.)Internet Email (Exchange 2010) -  

Adress Space SMTP * Cost 1
Network - User DNS MX records
Source Servers -  CASHT01 and CASHT02 (my 2 HT servers)

b.) SMTP - This is the one that was created by Exchange 2010 when the 2003 servers where found.

2.) Receive Connectors:

2 CAS servers:

CASHT01

Default CASHT01
Network - 0.0.0.0 - 255.255.255.255
Authentication - TLS , Basic Authentication, Exchange Server Authentication, Integrated Windows Authentication.
Permission Groups: Anonymous, Exchange users, Exchange Servers, Legacy Exchange Servers.

CASHT02

Default CASHT02
Network - 0.0.0.0 - 255.255.255.255
Authentication - TLS , Basic Authentication, Exchange Server Authentication, Integrated Windows Authentication.
Permission Groups: Anonymous, Exchange users, Exchange Servers, Legacy Exchange Servers.

Note: The .DAT file provides email address, port, SMTP IP address but it does not provide the account password.

Any idea what is preventing the email to go through? Anything wrong with the current configuration?

Thank you.