What needs to be done on the server to give Outlook access remotely?
As with all things in Small Business Server, it is best to make sure you go about this in an SBS-sanctioned way--I've gotten myself in trouble in SBS assuming that since I could get a feature working in the full product, I should be able to make it work on SBS no problem... SBS has many moving parts all installed on a single server which creates some unique interactions you may not see when deploying the same solution using multiple servers and the full licensed product!
Given that, I'd start with this Microsoft TechNet article which specifically addresses enabling RPC-over-HTTP on SBS 2003: How to Deploy RPC over HTTP for the First Time in Small Business Server 2003 (Standard or Premium)
[One of the things you may need to do (if you've not done so already) is to deploy an SSL certificate. You can go with a self-signed certificate or one signed by a recognized Certificate Authority (CA)... going with a self-signed certificate is the inexpensive route, but depending on your deployment and user community, using a "real" CA-signed certificate can save you some headaches. Should you choose to go that route, I'd recommend using DigiCert. They're not the cheapest, but are reasonably priced and their service and support is stellar; I wouldn't use anyone else.]