I've been using it on the last zillion messages and it has worked perfectly. You can set it low rejection time at 2-3 minutes to get 90% of the positive effect.
Or better yet, set it in read-only mode for a week before deploying it (e.g. prepopulate the database with sender/recipient/IP triplets.) Most of your normal traffic will have been prelearned and there will be no delay.
One (obvious?) point worth mentioning is that you should be whitelisting your own outgoing smtp servers (that is, if you've split up incoming and outgoing mail..).. The majority (at least for us) of (important) legimate mail is sent locally between our users..
Cami
