Lotus Notes 4.6 SMTP MTA / spam question

We use a Lotus Notes 4.6.5 server for our small business.  Lately, we've been getting spammed by someone who seems to be randomly mailing spam to thousands of randomly generated names at our domain which, of course, are not valid email addresses in the address book.  As a result, our server determines that the addresses are invalid and then attempts to return the spam email with an undeliverable message.  Of course, the return address isn't valid either so the server bogs down with thousands of messages in the outgoing mail queue.  As a result, legitimate outgoing email gets stuck in the queue for hours or even days.

I can think of three possible solutions, but can't figure out how to implement them:

1) Stop the server from trying to send bounce messages when someone emails an address that is not in the address book;
2) create a catch all mailbox for all emails that cannot be delivered to a legimate address which can be deleted periodically; or
3) send undeliverable mail to /dev/null if there is such a thing in Notes.

Can anyone help me accomplish this or a similar result with a minimum of pain?

I would hope that there is just a SMTP MTA switch that I could stick in the notes.ini file to accomplish this, but I just can't any documentation about it if it does exist.

Thanks a million.