That is because there is only one POP (with its own user and pass) account that catches all the email; the only way to retrieve your email with Outlook Express would be to access the catchall account as owner of that account, in that case you would be reading all the email of all the users (not a good idea).
I hope I am right
Sincerely
Arturo
I hope I am right
Sincerely
Arturo