Throttling by IP

/usr/lib/MailScanner/MailScanner/CustomConfig.pm contains a number of handy little features that are not “on” by default. One which came up on the list today was IPBlock which allows you to control the maximum number of emails from a particular IP per hour. This might be useful for combatting infected machines spewing email at servers, but it still does not address the issue of the connection to the MTA.
Vispan can also achieve this.
It would be interesting to implement something like this that interfaced directly with IPTables instead of the MTA.