Relaying Denied

From Mac OS X Server FAQ

"You're not a major ISP" is the likely reason. You may be hosting your mail server on a cable network, or on DSL or some other "home" or "personal" or "dialup" network that is NOT specifically designed for businesses. You probably have a dynamic IP address. These non-business IP addresses are purposefully blocked by major ISP's (eg: AOL) in order to stem the tide of spam. You'll need to forward your email thru a legitimate ISP (which is difficult due to that OS X Server GUI can't forward mail using any authentication) or get a "Business Account" from your ISP and get a real, static IP address.