Is this person in your Contacts folder? If so, open that Contacts item and
double-click the E-mail address. Examine the "Internet format" drop-down.
Does it say "Send plain text only"? If so, change it to "Let Outlook decide
the best sending format".
The recipient is in my contacts and I went in and it was "let Outlook
decide". I made doubly sure that was selected and tried again. It didn't
work. I've also changed it to be "Rich Text Format" but that didn't work.
Neither of us are on Microsoft Exchange Servers, if that matters.