@taazz
It is easy to detect spoofed emails even if you are under a dynamic IP. E.g. SpamAssassin uses the techniques I am referring to. That's because the mailserver is known, at least the outgoing last IP. And that is a real IP. Unless run your own mailserver under dynamic IP which has several other security related issues and is even not very easy to do.
Dynamic IP itself should be taken with a pinch of salt: usually it just means that your IP MAY change if the ISP feels it needs to. Normally it is stable for several months or in my case at home: years. Exception: some US providers change it a lot, but monthly, not weekly. The referring header is usually only a small part of serious anti-spam tools and works just indicative.
If you get blocked by spamassissin, your emailserver is in an IP block that is known to be used for spam. Your RH doesn't say much in such a case....
For running a mailserver under dynamic IP you need a dedicated service like
http://ip.thaddy.com (this example running by me for the last 10 years) to keep your mailserver configured properly. That's just to show you I know what I am talking about, but its use is free...