It's not a client, but it acts as a proxy between your standard IMAP client and the Web services interface provided by Exchange. I use it regularly with Apple's Mail.app. If you take a little extra time, you can also use it for calendaring with iCal and directory services with the Address Book app.
Hi,
One of our users has a eMac at home running OSX 10.2 using the default MAIL client. The Backend is Exchange 2003 with POP3 enabled. Our problem is, she can receive e-mail but she cannot send outgoing mail. The error is something along the lines of 'Outgoing Mail server 12.x.x.x.x cannot send mail' or something like that. Now we've seen the exact same problem on a windows machine with Outlook98 and Outlook 2003 and all we had to do was check off the option that says 'My outgoing server requires authentication' and then everything worked. I was on the phone with the tech support people at Apple, and they tried doing something similar under the OPTIONS and they made us type in her username and password under authentication. 