I changed my MX record that has a priority of 0 to point to my domain name. I didn't use my IP address, rather the name only (i.e. mydomain.net), and that has been working fine for me.
Since I have a dynamic IP address I have to use GoDaddy to send mail through so I am not blocked by some domains as a spammer. Somewhere on your email accounts page it will tell you the address to use for SMTP relaying. I believe I get 250 hops a day that came with the domain registration and can buy more if needed. You then enter that name in the SMTP Server section of the email settings page on your SME server-manager page. I don't think this step is necessary if you have a static IP...
Hope that isn't too confusing...
Ryan