Relaying based on Souce IP address

[pete pot]

Hi,
Currently my Postfix is realying emails from domains listed in $relay_domains 
only when those emails come from IPs listed in $mynetworks.
Is it possible with postfix to split this? to allow relaying some domains only 
when coming from IP 'A' and some other domains only when coming from IP 'B' 
Thx...
--PPot.