NT Connectivity Problem

Have several servers all running NT 4 w/sp6a. All are dual honned with 2 NICs. Nic #1 is for the internal network, Nic #2 is for external network. External network sits behind a Pipeline 130 router which gives internet connectivity for web, email etc. The problem is that on one machine I can get inbound access but not outbound. I can ping the ip from the outside. This is an exchange 5.5 mail server, and it will not send outbound messages. Will receive inbound. If I try to ping an external IP -I get an error message. Have checked DNS, WINS and gateways all appear correct compared to other machines. Any Ideas? Jeff