Depending on your version of Exchange, you can utilize Outlook Web Access. We’re using Exchange 2003 and for users external of the domain, they log into email witn OWA. It works fairly well. Everything has it’s limitations, but you don’t need a VPN just to get your email.
If I remember right, Exchange 5.5 and below do not have OWA.
Also if this is Exchange 2003 or 2007 you can set it up for RPC over http(s). That way users can use their outlook client over the internet as well if they do prefer that over OWA.