you can setup a POP3 account in their outlook as well as long as you have your POP3 virtual server started, and you have an A record traditionally labeled as mail.yourdomain.com directed to your mail servers IP address.
Make sure you have port 110 open on your firewall.
Or, you can setup RPC over http, which would allow you to use the full Outlook client from remote, without VPN’ing into the network. It is as if the remote users are on the network. Works really great!
See this doc for how to setup: http://support.microsoft.com/kb/833401