Reply text style is bold only when replying to one particular sender

When I reply to (or forward) emails from one particular person in our office, my text style becomes set to Bold.

I have confirmed that my Reply/Forward Stationery setting is Regular.  All other settings, font, size, and color are correct.

I also confirmed that the sender is NOT using Bold text when he sends email.  He is sending email in HTML format and I am replying in HTML format.  He is using Outlook 2003 and I am using Outlook 2007.

This problem only occurs when replying to/forwarding emails that this person originated.  When he replies to/forwards someone else's message, my subsequent replies/forwards obey all my Stationery settings.

What's going on?

Thanks.