error while receiving e-mails

When my remote user tries to receive his e-mail using Boardband connection, he gets following error message:

A time-out occurred while communicating with the server. Account: ‘Blow Joe
‘, Server: ‘106.19.xxx.xxx’, Protocol: POP3, Port: 110, Secure(SSL):
No, Error Number: 0x800CCC19

My mail server is behind the Watchguard Firebox Firewall and uses Dynamic NAT.

How can I fix this error.