I'm working on a proposal for a client of mine to reduce their software costs, and I'd like to provide the option of replacing Outlook with a free or low-cost desktop email client instead. I would prefer to use Zimbra Desktop over Thunderbird, but I cannot get ZD to connect to the Exchange 2007 server using IMAP. I fill out the account information, specifying that the SMTP server requires authentication. I include my username and password for the SMTP server, and ZD responds with 'SMTP server requires authentication'. Well, yeah, that's why I checked the box and put in my credentials!

I'm using internal IP addresses instead of attempting to resolve the hostname, and for right now I'm not even using SSL.
The sad part is that Thunderbird works fine with Exchange 2007 using IMAP/SMTP.

Am I missing something in my setup? I didn't really see any troubleshooting advice with a search of the forums.