This article was previously published under Q324642
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Macintosh clients who use a MAPI connection to their Exchange 2000 computer cannot connect when the Exchange 2000 computer is protected with Internet Security and Acceleration (ISA) Server.
This problem occurs because the ISA Server remote procedure call (RPC) application filter does not process the MSRPC requests from the Macintosh client correctly.
To work around this problem, Macintosh clients can use Post Office Protocol version 3 (POP3) or Internet Message Access Protocol, Version 4rev1 (IMAP4) for mail retrieval and Simple Mail Transfer Protocol (SMTP) to send mail. Macintosh clients can also use Outlook Web Access (OWA) as their mail client.
Microsoft has confirmed that this is a problem in Microsoft Internet Security and Acceleration Server 2000.