E2K: Problem getting SMTP-Mail

I hope someone around here can help me with the following situation:

New Windows 2000 Server with SP2, Exchange 2000 also with SP2

From the Dos-prompt:

NSLOOKUP set type=MX domain.com

shows the correct external IP of the mailserver, which is also configured in the firewall (One-to-One Nat: IP internal mailserver => IP external Mailserver). Port 25 for SMTP is open on the firewall as well as on the ISP side.

Sending mail with SMTP works fine internal and external, but receiving mails only internal. When I send a mail from the internet, I get the following error from the postmaster of the new server:

Repoting-MTA: dns;'mailserver.domain.com'
Received-From-MTA: dns;c000.ihr.cp.net Arrival-Date: Wed, 6 Mar 2002 15:45:45 +0100 Final Recipient: rfc822;'mailrecipient'

Action: failed Status: 5.0.0

Anyone has an idea what I'm missing?

Regards Henry