Problems Sending Mail to POP3 via Belkin Wireless Router

With my Belkin ADSL wireless router installed Outlook
fails to send email and gets an error, 'communication
with server interrupted' type message. However, using
the 'Test Account Settings' in tools, options, accounts
works fine, including sending a test email. So why is
this different from the regular way of sending mail?

If I switch the phone line to my regular USB ADSL modem
it works fine. PS Firewall is disabled on the Belking
ADSL modem/router.

Ideas?