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Why can't I send mail outside my organization?

I have my domain name as registered.xxxx.com. I have installed Exchange 2000. It's working fine internally, but when I want to send mail outside the organization, it does not go. I have already given MX record entry in DNS. What else should I do so that my mail can go outside the organization? If I send mail to any user with user.xxxx.com, it is sent and received properly. But if I send mail to a user at another domain, it won't go.
The purpose of a Mail Exchanger (MX) record is so external SMTP servers can look up and identify the mail server that is responsible for accepting e-mail for your domain. Since your problem is outgoing e-mail and not inbound e-mail, your MX record is not related.

It sounds like you might not have DNS working properly. Just as other servers need to resolve your MX record, your server needs to resolve the MX record for the destination domains. If your Exchange server cannot query external (e.g., Internet) DNS servers, it won't be able to locate the recipient's SMTP servers and, therefore, will not be able to deliver the e-mail.

In Exchange System Manager, if you right-click an e-mail queue (e.g., a domain name under the Queues object in the SMTP Protocol) and choose Properties, it will tell you towards the bottom of the dialog box why the e-mail message remains in the queue instead of being delivered. You might see, for example, a reason of "Unable to bind to the destination server in DNS." This means that your Exchange server could not locate an MX record for the destination domain.

Therefore, check and double-check your DNS settings using tools like Netdiag, Nslookup, IPConfig and Ping.

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