This is clearly a Quickbooks issue as it stems from the code that QB is passing to Outlook. But I know how devs like to shuffle the blame game. It's also irritating that this has been marked "solved" when it obviously has not been.

Moving on: as we have now experienced this issue on one of our users machines I was able to come up with another quick workaround that may help others:

1) Initiate email through QB

2) In the Outlook new message window click on the "Format Text" menu. Select "Aa Plain Text".
3) Compose your message and send

While this workaround limits some formatting functionality it will at least maintain your intended message.