While our antispam system is convenient, it makes decisions for you that you may not agree with.  The only way to avoid that is to shut it off for your domain (or for only selected users in that domain) and let you take matters into your own hands.  What do you do next, then?  We recommend the following tools:

SpamNet from CloudMark
This works for Ourlook and Outlook Express.  We were among the original beta testers of this service and know it to work very, very well.  We still have it on our own systems as a final-final antispam check.  Current pricing for new customers is US$3.99 per month.

K9
A full-featured and, more importantly, easy-to-use Bayesian content filtering system.  This is effectively the same system we use on our servers (conceptually, at least) and even includes the ability to create your own whitelists.  The software is donation-ware and a full-featured product is available at no cost.