[Serusers] problem with MESSAGE and un-REGISTER

hi all, I've started using SER 2.0 rc1 and am having a bit of problems with UA behind NAT. I'm using Windows Messenger 5.1 as the UA. Everything works fine when both UAs are not behind NAT. But when I place one of the UAs behind NAT, then, here's what happens, in this order UA Behind Nat ----------MESSAGE------>UA outside NAT..............ok UA outside NAT-----------MESSAGE------->UA behind NAT.............OK UA behind NAT------------MESSAGE------->UA outside NAT.............message is stored by msilo module and sent after some time. The log shows: MSILO: the downstream UA doesn't support MESSAGEs Notice that this only happens once a message from outside of NAT is sent to UA inside NAT. I am using RTP Proxy for NAT solution which is on the same system as SER. Moreover, I also cannot signout using the Windows Messenger. However, when I tested using XLite, None of the above mentioned problems arose. Can anyone please suggest me as to where the problem might be. The ser.cfg that I'm using is the one from the iptel site, the one with NAT & RTPproxy. I am also attaching the ser.cfg. Thank you in advance. Kumar