The environment: Exchange Server 2003 running on Windows Server 2003.
A few months back we sold a division with the majority of the HQ headcount. Part of the sale agreement required us to maintain the ability of existing customers to send to my mail server (email@example.com) and have messages automatically forwarded to the new corporation's mal server (firstname.lastname@example.org) while also responding to the sender to update their records.
Checking the agreements show that we have honored this clause longer than we had committed to (I've been busy.) Checking the Exchange logs shows that we forward about 18,000 messages a day. Checking those accounts that are most likely to get legitimate mail from outsiders shows that the top ten volume accounts have received just three legitimate emails sent to email@example.com in the last week- not an average of three a person, three total. The rest are malware, bot seeds, alternative pharmacutical promos, pr0n, unusual financial opportunities, and backscatter.
What I want to do is stop accepting this traffic, drop all the packets, and never bother to tell anyone since it will simply generate two levels of backscatter - mail bouncing back to a forged email address followed by that domain sending me NDRs for firstname.lastname@example.org. I'd rather set my exchange server so that it simply drops anything that isn't addressed to one of my valid users into the bit bucket rather than denying all inbound mail to the invalid addresses and generating all of those denials.
I did a quick search of the KB, but I'm clearly not asking it the right question since I'm not seeing helpful answers.
Suggestions other than migrating to sendmail?