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As long as you have an SMTP server listening on an IP that has a corresponding MX record, spammers will likely send emails to it, as you noticed already.

Your easier option, if you do want to keep the secondary(s) MX records for redundancy, is to place SpamFilter or another anti-spam solution on those IPs as well to block the spam there as well.

To prove that we don't want to sell more licenses :-) and if you want to give your programmers a bit of work, you could try the following:

Configure the SMTP server on the secondary MX to trust only the IP SpamFilter uses, so that only connections from that IP are trusted. This way internet users cannot send email to it directly. Write a small app that checks to see if the primary MX's IP is listening on port 25 (if nost the primary MX is down, spamfitler has crashed, the server has crashed, or whatever). If not, your app could configure on the fly your SMTP server to now listen on all IPs, not only the trusted on on the primary MX. It's a bit of work, and it may or not be possible to implement depending on your SMTP server, but it's an alternative if you don't want to place SpamFilter on all your secondaries.

Roberto Franceschetti
LogSat Software