Forwarding e-mail

I need to be able to forward email from our exchange server 5.5 sp3 registered as thayeraerospace.com to our sister company at aircap.com on a different ISP. Can I use the global address list to accomplish this so that our users here running outlook 97 & 2000 can just use the gloabal address list to send and e-mail to our sister company? If not how can I at least forward the e-mails that the clients send to the clients at aircap without setting up each individual client contact list?