Cannot send email from Exchange to internet

Hey all:
I have set up Small Business Server 2003 with Exchange for a client. I can receive email from the internet using POP3 connector all right, but I cannot send email. I have read that ISPs tend to block port 25 on their servers (we use bellsouth.net), so my question is this:
Can I use DNS to send email and what would I use as the external DNS server? Would it be the bellsouth.net DNS server? I am really confused as I can send emails from the server using Outlook express. I assumed that it would work the same way with Exchange…
Thanks to all…
Mike N