Using SBS when people don’t want a mailbox

I just installed Microsoft Small Business Server which includes Exchange, and some people in the office want Exchange mailboxes and some don’t. Does anyone know how to configure this? The problem is – if someone with an Exchange mailbox sends an e-mail to someone else in the office, the e-mail will land in their exchange mailbox and they won’t find it. Most people are happy downloading e-mail directly from the ISP into their Outlook and storing it as a pst file (because that way they can eg set up autoforwards to Google, and the SMTP is much more reliable when its run by the ISP than when its run internally).