Networks

In my company I have internal network with internal POP3 mail server.
On cisco ASA 5505 I made remote access vpn tunnel to use internal mail server.
When i do successful vpn connection with cisco vpn client, i open MS office outlook 2003 and try to send to any outside mail server message which is digitally signed.
When i check the mail box on outside server, the message is there, but without digital signature - smime.p7s.When i send mail message from internal network to outside mail, the digital signature is shown.
I think the problem is of configuration of ASA 5505 vpn tunnel.
Could you help me, where i have to look for solution of the problem, or how to solve it.
Thanks in advance!