For the last week I have been working on a smtp gateway. The purpose of this gateway is to take the load of of some of the other anti virus/spam hardware we have. So my boss has asked me to try and develop a postfix server that would catch all smtp traffic, look at the traffic to see if its a valid user account and forward it if it is. If not he wants Postfix to dump the message on the local machine.

After about a week of work I was able to creat a catch all by doing:

/etc/posfix/main.cf
virtual_alias_domains = <internal email server (MS Exchange)>
virtual_alias_maps = hash:/etc/postfix/virtual

/etc/postfix/virtual
@<mydomain> root

As of right now that works great, but thats about it. What i want to do is add the valid user accounts to the virtual file and as it checks to see if a email is to one of these valid users to forward it to the exchange server. can this be done?

Well you'd not primarily be looking at using a virtual *file* at all, but hooking directly into the ldap interface to your AD domain; http://www.postfix.org/LDAP_README.html Hopefully there's enough there for you know, if not to directly implement in your environment but to see that a real hands free slick solution is really achievable.