allow from only one IP

hello,

I need one thing and don't know the answer. On one account I've got enabled 
forwarding. Now, I need to enable that forwarding is done in only one case. 
1)If mail comes from IP1; then forward
2)If mail doesn't come from IP1; then discard 

In mta logs I see header "fromhost", but it doesn't work...So, how to set rule 
to achieve that?
thanks in advance

LP