Exchange Server accessing Problem

We have exchange Server 5.5. We don’t have access problem within LAN or Outlook web access. But pop up server and SMTP configured outside LAN can send mail but they can’t download mails. What could be the problem.

When using MS Outlook to download messages we get the following:
‘Delivering Messages – checking for new messages and sending messages’.
Then the message……’Internet E-mail – The POP3 Server name cannot be found – mail.ourserver.com’.