If the Domino server is running the IMAP task (or POP3), then the Outlook Express client can retrieve mail from the domino server. This is simple to setup if the user has access to the server but this may require a modification to your firewall specifically for that user if you don’t want to open up the port for everyone.
The password to enter into Outlook Express is the same as the browser password (i.e. the one stored in the address book).
The alternative of course would be just to add an agent to the mailbox to forward the mail to his private address.