Make sure the default email from address is configured correctly in Administration > Site Settings
Your email account probably requires you to send it from a specific address or at least a valid address for the email domain.
Other than that I would refer you to your host for help, possibly they may need to configure the mail server to allow relaying from the web server.
Note that only the user.config settings are needed, the ones in Web.config in the system.net section are no longer used in mojoPortal so we don't need the settings in 2 places anymore.
But when you edit settings in user.config the runtime will not automatically detect changes to that file so after editing user.config you need to touch Web.config to make it reload settings, ie type a space in it and save it.
Hope that helps,