Access Exch. Svr. 55 mail from Internet!

Hi! I have an Exchange Server that is on the inside of a Firewall. I also have one SMTP server that is on the Internet (outside the FW), that receives all the mail directed to my DNS wich redirects it to the Exchange Server. The only machine visiblefrom the outside is obviously the SMTP Server.
Can anyone tell me if, with this current configuration, I can access the mail accounts, or what do I need to do so? Where can I find documentation for support on this?