Question : How to configure email for authenticated users + some domains

I want to change the settings of delivery restrictions > message restrictions.
currently it is configured to accept messages: From authenticated users only. In this way we can only receive emails from authenticated users who are part of our domain.
Now there is a requirement that we have to receive emails from one external source [email protected]
How can i configure this setting for organization wide. there are almost 3 thousand users. i need a solution in which least administrative effort shoud be done.
Environment detail:
2003 DC
2003 Exchange

Looking forward to hear

Answer : How to configure email for authenticated users + some domains

Here's some videos that might help: http://screencasts.ubuntu.com/SAMBA_Filesharing

Here's some pretty detailed instructions: http://ubuntuforums.org/showthread.php?t=280473