[Serusers] SER on multihomed host

Hi, I am running SER on a host with 2 network interfaces. The behavior I observe is that when SER receives a message say REGISTER on interface 1 it sends the 200OK on interface 2. Due to the way the network has been configured, this 200OK never makes it back to the UA which sent the REGISTER. So I want to make sure that SER sends the response back on the same interface it got the message on. How do I do this? I cannot disable the second network interface. In ser.cfg I set "listen" to the IP address of the interface I want to use; I also set "mhomed" to 1, still no luck. Any suggestions are highly welcome. thanks, -Ravi.