Postfix setup help required

Hi guys this is my situation ;

I have an out of the box installation of fedora core 5

I need to set an SMTP server that will accept mail for sending to any
domain as long as the from address is something@mydomain.com
I do not want SMTP autorisation.

I would like the following help .....

1) configure postfix from scratch to achieve the above
2) Ensure that I am not an open relay
3) I have filled out the the appropriate TXT values in DNS as follows

mydomain.com TXT - v=spf1 ip4:99.99.99.99 mx:mail.mydomain.com -all

Is there anything else I should do to ensure that other mail servers
don't think I'm spamming.

Thanks
D