[Serusers] Sip server

Hi, I am new to SIP. I have installed the SER server and my REGISTER method is working ok. But when I send the INVITE method I am getting the following error from SER server. The invite packet which has been sent by me is: INVITE sip:anuradha@there.com SIP/2.0 From: <sip:LittleGuy@there.com>;tag=1044365919410 To: <sip:anuradha@there.com > Via: SIP/2.0/UDP 192.168.1.9:5060;branch=z9hG4bK1159de0c6cca18f9131c8ff342527668 CSeq: 1 INVITE Call-Id: a1371c790546f9a2df76be1f59f6a7dc(a)192.168.1.9 Contact: <sip:192.168.1.9:5060;transport=UDP> Content-Length: 0 192.168.1.9 is my SER server's address The response got from the SER server is: SIP/2.0 500 I'm terribly sorry, server error occured (1/SL) From: <sip:LittleGuy@there.com>;tag=1044365919410 To: <sip:anuradha@there.com>;tag=d907c037823644515dfe0ede38ca9976.3592 Via: SIP/2.0/UDP 192.168.1.9:5060;branch=z9hG4bK1159de0c6cca18f9131c8ff342527668;received=192.168.1.21 CSeq: 1 INVITE Call-Id: a1371c790546f9a2df76be1f59f6a7dc(a)192.168.1.9 Server: Sip EXpress router (0.8.10 (i386/linux)) Content-Length: 0 Warning: 392 192.168.1.9:5060 "Noisy feedback tells: pid=1587 req_src_ip=192.168.1.21 in_uri=sip:LittleGuy@there.com out_uri=sip:LittleGuy@there.com via_cnt==1" Can you help on this please Thanks in advance Geetha