Belkin Router Settings

I should just add that between the " " is a request from a spam filter service

It's already blocked by default.  The Belkin probably only has simple block and allow without settings for specific IP blocks.  You'll have to open port 25 to the internal address of the device in question.
Depending on the model of Belkin it will say something like network servers or port settings or something like that.

Hi Thanks, under firewall settings I have virtual servers but cant add IP address just port numbers

Sorry there is also these options under the firewall settings. Client IP filters, MAC Address filtering, DMZ, DDNS< WAN Ping Blocking, Security Log

You'll have to have a much more expensive router to do IP filtering.  The options you mentioned don't have anything to do with what you want.  DDWRT adds a lot of high end features to cheap routers if it will run on yours.  I don't know if it will add what you want though.  Check their site for your model and instructions.  http://www.dd-wrt.com/site/index

So in summary, I cant do it without firmwaring my router or buying a more expensive one?

what about through routing & remote access

Sorry I have to re-think my question