exchange 2000

Hi,

I have set up exchange 2000.
I am having the following trouble. Everybody is able to send email to everywhere in the world. But the world can not send back. They are not receiving emails, after a day or 2 they will receive a ndr message error code 4.4.1

I am able to telnet into pop3, smtp imap and everything and the nslookup is fine also.

What am I missing.

regards,

Leopold