How do I set up Exchange 2007 to allow POP/IMAP clients to send email?

Hello!
I have set up a SBS2008 server and am trying to get my Outlook to send emails when connecting as a POP/IMAP account instead of Exchange. Here's what's happened:
* I set up Outlook 2007 to connect as POP3 and can download message with no problem when accessing remotely.
* For the SMTP settings, if I deselect "My outgoing server requires authentication" and I set the outgoing port to 25 with "None" selected for Encryption type, the 'Test Account Settings" succeeds. I can also send to internal email addresses (same domain as mine). Any external address, I receive an immediate "550 5.7.1 Unable to relay" message for the Outlook SysAdmin.
*I think it might also be worth mentioning that when I'm connected to the network directly at the office, I can send to any address via this Outlook account BUT I must select "My outgoing server requires authentication" to do so, otherwise the messages do not go through.

So, I guess my question is what all do I have to set up in Exchange to and in Outlook to get this to work? I looked at the EMC\Server Configuration\Hub Transport\Receive Connectors\Default SBSServer Properties and saw that on the Network tab, the IP Address to receive mail is the server's IP and the port is 25 and all the authentication types on the Authentication tab are enabled.

Any suggestions or guidance would be GREATLY appreciated!