External email always bounces -outgoing

I believe I have everything correctly configured in Exchange 2000 SP3 to deliver outgoing messages to external email addresses but they always bounce. There is not problem receiving email and can send internal email without problems. I can ping and tracert external addresses without problems. The only thing I can think of is I have misconfigured SMTP or DNS. I have already read every Microsoft TechNet article on this and have tried all their fixes and tips. If anyone has ever seen this happen or could give me some ideas on what to do, please let me know. Thanks.