Not really a fix, more of a possible work-around. Sometimes you can get this error if you are using Microsoft Word as your message editor. Here is how to check, and change that setting:
On the Outlook menu bar select Tools > Options
Click on the Mail Format tab.
Make sure the “Use Microsoft Office Word 2003 to edit e-mail messages” checkbox is not selected.
I have not found out why this is a problem (yet) and I have a few users who ARE using Word as their e-mail editor with out any problems.