HI.
Allowing OWA to your corporate servers isn't very secure, since you are relying on IIS .
You should not use OWA unless you must.
You should limit and/or hide this service, or disable it.
Here are some ways:
1) Use VPN (PPTP or better IPSec) for remote clients.
Do not allow full access to VPN users (do NOT use sysopt connection permit-ipsec on the PIX) but instead limit VPN users only to specific servers and ports, like OWA .
2) If not using VPN, it is recommended to change the default port 80 to a different port.
This change must be done on both IIS server and in PIX configuration.
This is quite good against IIS atacks like code red, but it is not good against malicious attackers that scan your specific servers.
3) In any case, you must apply all IIS related patches on all servers running IIS, and disable IIS on critical servers that do not need it.
About your problem, I also think that a STATIC command is missing in your PIX configuration.
Provide more details go ger better answers.
Bye
Yizhar Hurwitz