We have an employee who technically is the entire staff of a subsidiary. He's using a Dell laptop identical in hardware and software to our regular employees. The problem is, when we send him an e-mail in HTML format, his Outlook displays it as Plain Text.

Now the only difference here is that he is using an IMAP account from 1and1, while the rest of us are using internal Exchange. I have verified that his setting in his Outlook is not enabled, to display all messages in Plain Text.

I'm at a loss to explain what is going on, so I'm about to open a ticket with 1and1.