Creating a list of about 1200 mailing addresses

I work for a certain company that has tasked me to create a list of about 1200 mailing addresses. Being familiar with Outlook DL I am aware that this is much more than it can handle. This list can be handled well by Exchange 2003. However, I don’t know how to come up with this list considering that I don’t have these users in my AD and would also like to avoid a situation whereby I would be forced to enter all the 1200 external addresses into my AD. Someone please help. Thanks.
