More than likely the issue is that the new Exchange Server does not allow mail relay from the Aardvark server, which is the default, and very sensible setting. You should get the Exchange Server admin to allow relay from the IP address of the Aardvark Server. There are other potential configuration issues, such as the SMTP server requiring authentication.
The Exchange server itself may also reveal the reason why you cannot send mail. If you open a command prompt on the Aardvark server, then run ‘telnet <name of exchange server> 25’. This will allow you to run SMTP commands on the Exchange Server. If the connection is successful, try sending a test message using these commands:
MAIL FROM:<your email address><return>
RCPT TO:<your address><return>
You should get the email in your inbox. If you do not, please let me know.