Exchange question

I have a win2k exchange server, I am having problems receiving internet email. I can send and receive email fine on the local network. I can send email from the exchange server to the internet. I can even use a mail account through my ISP to send email to the exchange server, and the email will be delivered. I cannot receive any other internet email.
I own the domain name, have configured the zone records-mx and A records.
Nslookup seems to show the correct information, I can telnet my smtp server, can ping the domain names. It appears connectivity is not an issue. I think that the mail is being lost at the name servers. I suspect the MX records although they seem to be configured correctly.
Anyone have any Ideas?