Your best bet to make Outlook behave better as an IMAP client is to configure a mail "profile" via Control Pannel --> User Accounts --> Mail, and set all the particulars there. Recent versions of Outlook have a stripped down configuration interface that offers no flexibility. For example, from Outlook itself it's not possible to set an IMAP login name that's not an email address.
Yes, this was a "holy shit" moment that I had today. I couldn't even see how to change the user name. Outlook has got to have the worst, most inconsistent user interface for a mail client I've ever seen. It's insane.
Thanks for the tip on the Mail settings. I wasn't aware of those.
I bit the bullet and got a free trial of MS Outlook as part of Office 365 so I could do some testing. It was super easy to set up and I had absolutely no issues logging into my client's IMAP account with. I spent an hour with the client today, who had a slightly older version of Outlook, and we could not get it working. It took 5 minutes just for Outlook to fail and finally tell us it couldn't log in.
As I think about this, it's probably some kind of encryption protocol issue. Is it possible some older versions of outlook are using outdated encryption methods that my server is not set up to work with?