Do you mean you want to have Postfix store mail for Outlook users to access? That's no problem (it's well covered in the docs), but what you also need is a POP3 or IMAP server with which Outlook (or any normal mail client) can communicate; its job would be to retrieve mail from the mailboxes where Postfix stores messages, and send them to the client on request. For example, Courier-IMAP is a good POP/IMAP server.
You might also want a POP client which would regularly download messages from your ISP to your own server. Getmail and Fetchmail are popular for this job.
