One of my co-workers just got a new Lenovo laptop with Windows Vista Business preinstalled. Shortly after he started using it out in the field he reported problems composing and responding to messages in Exchange 2003′s Outlook Web Access. I set him up with Outlook 2007 in cached mode, so that bought me some time until I could get some time to work on the issue.
I finally found the solution to this problem this week, in the form of an Exchange hotfix described in KB 911829. This hotfix installs a new message editor that uses an Internet Explorer “iframe” instead of an ActiveX control. Windows Vista no longer includes support for the ActiveX control that is used for HTML editing in Outlook Web Access.