Can’t get Exchange Server to Send Email using DNS

I have a new installation of SBS 2003 with Exchange Server. I can receive email with the pop3 connector, but am unable to send emails using DNS to route email. I can send email using the “forward all e-mail to e-mail server at your ISP”, but this, I believe, can result in some of the email I send being treated as Spam as it will have been been forwarded. Currently I am I am unable to send to hotmail accounts for example.
If I am only using DNS routing to send email is it necessary for me to have an MX record and/or a fixed IP address?