Traffic goes viceversa on the same interface..

I have linux box, in that having 4 interfaces ,each interface is using a different ISP Links...
now i seperated whole network in our lab as three netwoks as 32 its going via "A" ISP and 64 network needs to go via "B" network and 96 network goes via "C" network..
According through this network routing table is also working.. and default  0.0.0.0 is going via "eth0"
ie.,  32 network...if i want to change any network as default network i can change through the command..
Now my question is:
if any thing coming via through the interface "C" from the outside through port no "25" ,i need to send reply also in the same interface ("C").. for this what lines i need to add in the iptables.. or is there any alternative solution for this.. if have any code please forward me ...

==Expertblr.