Exchange 2000

I’m new to Exchange, so please forgive the broadness of this question. We have 2 mail servers in our company. One is running Exchange 2000, and one is running Rockliffe. The Rockliffe mail server works. The exchange server was brought in from a different office location. It used to work. Now, users can send out email, and recieve internal email, but nobody is recieving external email. Can somebody point me in the right direction? Thanks so much.