[Mimedefang] thoughts on "nolisting"?

[Gary Funck]

We've grown a bit frustrated by the amount of spam that flows
through our backup MX server.  Was recently reading about the
"nolisting" technique,  http://en.wikipedia.org/wiki/Nolisting
where bogus MX servers (primary and highest numbered backup)
are set up, in the hopes that dimwitted spam clients will
bounce off those bogus servers.

Anyone tried this?  Opinions on pros/cons?

BTW, it seems to me that more ideal backup MX behavior
might be for the backup to poll the primary, every minute/so,
and only open port 25 when it determines that the primary
isn't available ....

thanks.