Do you make access to group mailing aliases within your company or domain available to recevie mail originating from outside of the domain?

Most do, just for the fact that it is more work to close it off, and some people may not know how.

And be careful of doing this. Most legal departments could probably come down on you under some spam law somewhere, and possibly fine you for every user who received the spam.

Personally I believe spam needs to die. I get so much of it, and I ensure I never support the business advertised in it, nor do I respond. A backup job of mine right now if I lost my current job would be to move to a state with severe spam laws, and start suing under that law. I have heard of a few people doing this now, with pretty decent success. Most choose to pay the fee out of court instead of allowing it to escalate.