This is explained in the documentation about email configuration.
In the latest version of mojoPortal you don't need to worry at all about the <system.net section, it is no longer used.
You certainly should not copy that section into user.config it would cause an error.
user.config is only for overriding the <appSettings section of Web.config nothing else from web.config can be put in user.config and trying it would only cause errors.
Hope that helps,
Joe