[dba-Tech] Small Business Server 2003 and smartphones

I have a SBS 2003 server running Exchange that I need to allow some of our users to get their e-mail from on their smartphones.  I'm using a Sonicwall firewall appliance between the internet and our network and with a dynamic IP address.  I can use DynDns or no-ip if I need to have a static ip but what settings to I need to make on the firewall and server to allow users to securely connect?

Any ideas or links would be appreciated.

Thanks,

Rusty Hammond