While Exchange has protocol support for POP3 and IMAP clients, it does not natively provide connector services to POP3/IMAP4 servers as a client. There are software vendors who advertise solutions to this, although I have no first hand experience using them. Products include Native POP3 Connector (NPC) from MapiLab, PYTHEAS MailGate, or POPBeamer for Microsoft Exchange from DataEnter.
As you are probably already aware, you can easily configure each Outlook client to connect via POP3 or IMAP4 to your ISP, in addition to the account connection to Exchange.
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